For what it's worth, I've seen none of the symptoms described here. I use WiFi at home with a Linksys WRT160N Wireless Router behind a Netopia/Cayman 3346 DSL Modem/Router/Firewall (I subscribe to BellSouth/AT&T DSL 6 Mb/s). I've experimented with trying various things with the WiFi off, seeing how it performs with EVDO (so-called "3G" mode; no "4G"/WiMAX in my area (Ft. Lauderdale, FL, US)), then turning on WiFi to compare performance. Other than speed differences, I've experienced NO differences in functionality. Things work (or don't) the same using either connectivity (other than access to nodes on my LAN that are inaccessible from outside the firewall).
The port-blocking for SMTP (port 25) by BellSouth (now AT&T) has been going on for several years, so I use SSL authentication/encryption which uses port 995 by default. This port is *not* blocked, so it's all happy.
I'm on a Sprint/HTC EVO with the OTA update applied to 1.47.651.1
BTW: I strongly recommend setting this up on a mobile client, because, by default, all SMTP (port 25) traffic is "in the clear". Anyone packet-sniffing can read your (supposedly) private Email you're sending.
Similarly, if you're using POP3 (port 110) for Email retrieval, then (most likely) all of your retrieved Email is also open to be read by anyone snooping on the same network as your mobile device (WiFi or office LAN).