That was me posting about the 56Kbps cap on the at&t forums that you had quoted ....
Even if you are using WiFi for data, that 56Kbps cap causes other problems:
1) Even background data like location services or visual voicemail can use enough bandwidth to cause stuttering in your voice phone calls, and some people find they have to turn 3G data off in order to make voice calls without stuttering.
2) MMS always goes over 3G data, even if there is WiFi, and as others have noted, it's very slow, and sometimes times out completely because it's so slow.
I strongly suggest anyone with a Microcell call and complain about it, get a case #, hopefully they will realize they need to fix it. The direct number for AT&T's "tier 2/3" tech support is: 866-246-7344