Ross Dembecki, Microsoft CRM Lead Product Manager (Australia) has posted a PDF on his Windows Live 'SkyDrive' to help address this question.

The first thing you need to know is which licensing program, or channel, are you acquiring your CRM license under?

To help you understand the "options" and then where you need to look, Microsoft has published a table with a column for each program that you may be getting CRM licenses under, which Ross has kindly made available as a PDF here......

CRM 4.0 License Keys table

Please be advised that you should not install Microsoft Dynamics CRM 3.0 Update Rollup 3 (UR3) on any sites as it has been withdrawn due to a regression discovered in one of the included fixes. 

The issue – After applying UR3 or one of the specific hotfixes listed below, when opening an existing Service Activity and clicking on the Customers field to drill down to the Customer information the following error is received:

Record is Unavailable. 

The record that you are requesting is currently unavailable.  Either the record was not found or you do not have sufficient security permissions to view it.

Microsoft’s Sustained Engineering team for Microsoft Dynamics CRM are working on a fix for this and will re-release UR3 as soon as possible, no further information on timescale available at this time.

The following individual hofixes are also affected by the regression and are being withdrawn as well, for these a new single hotfix will be produced to replace them...

949106  Error message when you add a contract detail to a contract in Microsoft Dynamics CRM 3.0: "The start or end date is not valid"

http://support.microsoft.com/default.aspx?scid=kb;EN-US;949106

943417  The number of campaign responses and the owners are incorrect in the Advanced Find view when you use the Owner column to sort the campaign responses in Microsoft Dynamics CRM 3.0

http://support.microsoft.com/default.aspx?scid=kb;EN-US;943417

942010  A user can run the Mail Merge function even if you remove the Mail Merge privilege from the role that the user has in Microsoft Dynamics CRM 3.0

http://support.microsoft.com/default.aspx?scid=kb;EN-US;942010

941592  A request to the Reporting Services Web site is unexpectedly triggered when you open an entity form in Microsoft Dynamics CRM 3.0

http://support.microsoft.com/default.aspx?scid=kb;EN-US;941592

948279  Information about the March 17, 2008, update to the Help files for Microsoft Dynamics CRM 3.0

http://support.microsoft.com/default.aspx?scid=kb;EN-US;948279

946722  The sent date on the original e-mail message is incorrect when you reply to an e-mail message from the Microsoft Dynamics CRM Web client

http://support.microsoft.com/default.aspx?scid=kb;EN-US;946722

946130  Error message when you create a related record from a custom entity in Microsoft Dynamics CRM 3.0: "Insufficient Permissions. The logged-on user does not have the appropriate security permissions to view these records or perform the specific action"

http://support.microsoft.com/default.aspx?scid=kb;EN-US;946130

946128  The date in the Date Fulfilled field is the default date even if you enter a different date in the Fulfill Date box when you run the Fulfill Order function in Microsoft Dynamics CRM 3.0

http://support.microsoft.com/default.aspx?scid=kb;EN-US;946128

946070  Error message when you try to save an instance of a custom entity in Microsoft Dynamics CRM 3.0: "Invalid Argument"

http://support.microsoft.com/default.aspx?scid=kb;EN-US;946070

945220  Microsoft Dynamics CRM 3.0 does not create an e-mail record when you send an e-mail message to a Microsoft Dynamics CRM user

http://support.microsoft.com/default.aspx?scid=kb;EN-US;945220

944256  You can delete a Subject record that is used by a Sales Literature record in Microsoft Dynamics CRM 3.0

http://support.microsoft.com/default.aspx?scid=kb;EN-US;944256

944041  You cannot sort relationship views by using a column after you install hotfix 924882 in Microsoft Dynamics CRM 3.0

http://support.microsoft.com/default.aspx?scid=kb;EN-US;944041

941862  A user who has the Assign Role privilege in a child business unit can unexpectedly assign the System Administrator role in Microsoft Dynamics CRM 3.0

http://support.microsoft.com/default.aspx?scid=kb;EN-US;941862

940761  You cannot process an activity that you are viewing in Microsoft Dynamics CRM 3.0

http://support.microsoft.com/default.aspx?scid=kb;EN-US;940761

940496  Error message when you try to view activities in Microsoft Dynamics CRM 3.0: "A SQL Server error occurred. For more information, contact your system administrator"

http://support.microsoft.com/default.aspx?scid=kb;EN-US;940496

939680  You can merge a contact, an account, or a lead by using a custom SDK application even though you do not have the Merge privilege in the security role in Microsoft Dynamics CRM 3.0

http://support.microsoft.com/default.aspx?scid=kb;EN-US;939680

938358  Error message when you perform an action in Microsoft Dynamics CRM 3.0: "You do not have the required privileges to complete the action"

http://support.microsoft.com/default.aspx?scid=kb;EN-US;938358

915520  An e-mail message is not delivered to Microsoft Dynamics CRM 3.0

http://support.microsoft.com/default.aspx?scid=kb;EN-US;915520

945311  A hotfix is available that adds the new UTC -4:30 time zone for Venezuela in Microsoft Dynamics CRM 3.0

http://support.microsoft.com/default.aspx?scid=kb;EN-US;945311

940715  A queue item is displayed in the Assigned queue of the last user who accepts the queue item in Microsoft Dynamics CRM 3.0

http://support.microsoft.com/default.aspx?scid=kb;EN-US;940715

If you have installed either UR3 or any of the individual fixes listed above then please open a support case for further direction.