After migrating users from Exchange 2003 onto our 2007 cluster, we found that attachment viewing stopped working in ActiveSync.
The fix is to delete the ActiveSync profile on the phone (and maybe on the server, I did this via Outlook Web Access just to be sure) and then create a new one. There is a security policy on the CAS server end which gets set up when a new device connects, our theory is that if the phone doesn't make a new connection to the Exchange 2007 server, that security policy never gets pushed to the client and the server will refuse to download attachments to the phone.
You also have to allow attachments in the Default ActiveSync Client-access profile on the server (but this is enabled by default).
So, if you have users who say they get errors when downloading attachments to an ActiveSync phone, nuke the profile on the phone and recreate the association, it should then start working once again.