We just did a quick check for a client investigating the possibility to upgrade their BizTalk 2006 platform to the latest (2010) version. The main issue was whether the vast amount of integrations to their Ax 4.0 system would still work.
Some of the integrations are made using the Ax AIF adapter in BizTalk and while their is a lot of information on the web that doesn't say that BizTalk 2010 can work with the Ax 4.0 adapter, there is not a single document that explicitly says that the two are incompatible even though you more or less can assume so based on the information available. Mainly because BizTalk 2010 requires Windows Server 2008 and the adapter is not supported on this platform. The adapter is not supported on 64-bit systems at all and I doubt there is many admins looking at installing a fresh new integration platform today and not putting it on a 64-bit Windows.
We checked with Microsoft just to be sure and received the answer that BizTalk 2010 and Ax 4.0 cannot be integrated using the adapter. Not even Ax 2009 is fully supported by BizTalk 2010 as of today which is an interesting fact. Instead are we investigating the amount of work needed to change the adapter based flows to MSMQ integrations instead. The upgrade of the integration platform is more important than to keep an outdated adapter working..