If you are a MS CRM person looking at SharePoint as something new here are a few things that may be of interest.
The SharePoint 2010 Foundation is free and will work with the MS CRM list component but it is limited to database sizes of 4Gb. This makes it much easier to leverage some of the power of SharePoint for your CRM customers. Your customers by default already have licensing for Windows Server and SQL Server in order to run MS CRM and SharePoint Foundation can share that same server.
In addition the new Business Connectivity Services (BCS) are also supported by SharePoint Foundation: http://msdn.microsoft.com/en-us/library/ee557646.aspx
BCS will allow you to expose data from other databases to show up in SharePoint, but not take space in the SharePoint database, this of course includes CRM data. There is obviously a performance penalty depending on the interface to access this external data, but it also allows maintaining legacy databases while having access to that data in SharePoint without passing the 4Gb database limit in the SharePoint database.
If you are looking at SharePoint integration using their web services, you can see from the following link which services are supported by both SharePoint Foundation and by SharePoint Server and just by SharePoint Server. http://msdn.microsoft.com/en-us/library/ee705814.aspx
As an obvious guideline it would make sense to write your base code to support SharePoint Foundation to let customers try out SharePoint functionality, and to have any features that require SharePoint Server features be switchable at a later time if your customer upgrades.
SharePoint Server has a number of useful features shown in this version comparison that you may or may not need: http://sharepoint.microsoft.com/en-us/buy/Pages/Editions-Comparison.aspx
As a practical note, SP 2010 is built on .NET 3.5 and CRM 2011 is built on .NET 4.0, so while most of you will not have a lot of .NET 4.0 legacy code at this point, is will be important to make sure your libraries with logic that will be shared between SP and CRM components are all kept at .NET 3.5.