A couple months back I did a test for a client about whether the legacy Dynamics CRM 4 adapter would work successfully on BizTalk 2010. I know this is not the recommended configuration and that the CRM 2011 SDK does not mention this as an acceptable or supportable configuration. Many companies have made a significant investment in CRM 4 and may not be able to upgrade their CRM software. So I am just putting this information out there in case you are wondering.
Others have reported needing to use a registry workaround for getting the adapter to install for BizTalk 2009: http://geekswithblogs.net/BizTodd/archive/2009/05/12/132058.aspx. I followed this workaround with BizTalk 2010 on Windows Server 2008 R2 and it worked fine. The CRM 4 adapter install worked fine. I was also able to run the schema creation wizard in VS 2010.
I did not try running the software in this configuration in a production environment and have not tested this configuration completely. But usually in my experience if the install works fine and the designers still work in VS then you can probably work with this configuration, if only for a temporary workaround while you create a migration strategy.