About XE2, the only possibly breaking new feature shall be 64 bit.
But from the COM point of view, it may matter for a COM object to be implemented or called in either a 32 bit or 64 bit code.
COM object types are registered via registry keys. This is what the
regasm.exe utility does (writes to the registry). 32-bit bit registry
nodes and distinctly different than 64-bit registry nodes.
For instance, if your COM object is compiled as 32 bit, it shall be registered in the 32 bit registry or the 64 bit registry (via reagasm
calls)
But if you stick with 32 bit Delphi application, you won't have any additional issue, if you specify the COM wrapper to be exported as 32 bit. In short, XE2 behaves exactly the same as any previous version of Delphi, when creating 32 bit processes.
See http://www.gfi.com/blog/32bit-object-64bit-environment for additional information.
Another solution could be to run the COM object out of the process: in this case, the bit-ness won't matter, and you should not have any issue about 32 or 64 bit.
Be aware that there was a very annoying bug in XE2: in short, COM access was broken in some cases. Sounds like latest "Update pack 2" solved the issue.
Of course, if you use the cross-platform features of Delphi XE2, your COM object won't be available in Mac OS X. ;)