Sprint/Verizon and all other CDMA carriers have that problem. It has nothing to do with the device itself. It's the network's abilities, or lack thereof to prioritize voice calls over data. GSM is better at pausing a data session to allow a call through. The current wireless technologies do not support simultaneous voice and data connections...it's either or at any one point. When you're on a call, you cannot send or receive email or run any other data programs during that call.
Looking into the future, this will be supported, but don't hold your breath. UMTS, which is what Cingular is trying to figure out, will offer that ability. CDMA EV-DV will offer that, but that standard will not even be completed until 2006 or 2007 at the earliest...if it ever does get created. Keep in mind that just because the network may be able to support it, devices will need the same technologies built in. I wouldn't expect this functionality until several years out.
Sorry if I get a little "geeked out" here, but rest assured that you're not the only experiencing this. It is something that everyone using a converged device has experienced at one time or another. That is one of the reasons I carry two phones...a regular cell and a BlackBerry and use the BB for voice/data, but if I'm on a long conference call, I use the regular phone so I can still send/receive emails and access corporate data.
Current - 8700g TMO
Previous - 7280 AWS, 7730 TMO, 7290 TMO
Long ago - 950 model (still have it)