You can use the ACS server at the main location. The only problem that I can think of that may be an issue would be caused by latency of the link back to the main site. You may need to adjust your radius timeout values if the links have high utilization or an latency issues.
The other thing I would do is implement WDS at the remote sites. This will cause the first authentication to get passed to the ACS server and subsequent authentications to occur locally to the WDS master AP at the remote site for the specific account.
I have a similar setup at many of my remote sites. All of my remote sites connect back to the main site via T1's that have AP's deployed. I have not heard of any problems from my users with this setup.
HTH
Steve
