Found this one out the hard way this week; it seems that currently the only supported version of SSRS for the SharePoint 2010 SRS Add-in is SQL 2008 R2. At least one forum poster from MSFT has confirmed this.
Annoying as it can be surprisingly hard enough to convince customers to install Windows 2008 let alone upgrade their SQL servers! Ah, well at least SRS can be separated from the SQL DB servers making it a bit easier.
Update 12/07: This has now been addressed by Microsoft in the SQL 2008 SP1 CU8 release which adds support to reporting services 2008 to work with the R2 add-in.