I just spent a long time troubleshooting a problem receiving MMS messages after installing Handcent (verizon motorola droid 2). MMS messages weren't coming through either on Handcent, or on the stock text messaging app.
I found that I needed to enable settings in two different places.
1 - the auto-retrieve setting in the main android setting
2 - the auto-retrieve (and roaming auto-retrieve) settings in the Handset app itself. It seems that the Handcent settings override the main android setting (and, affected receiving mms messages in the stock app ???) so these need to be tweaked. For some reason the defaults were off.
Get to the handcent settings from the main handcent listing view, menu, settings.
Anyway, this worked for me.
HTH.