Sprint (and Verizon) are CDMA carriers.
Sprint and Verizon CDMA networks currently segregate voice and data channels. Voice transfers over the 1xRTT channels, whereas data transfers over the EVDO (Rev. 0 and A) channels. This measure was taken for the purpose of load-balancing. Given the finite amount of spectrum, it is logical to operate voice over a separate channel for the purpose of reliability and consistency.
This is one of the reasons AT&T has an inordinate amount of dropped calls. This is why AT&T is scrambling to overlay additional spectrum (850MHz). AT&T is also in the process implementing a load-balancing system (N-SET) with the intention of decreasing dropped calls and slow data sessions.
It is theoretically possible for Sprint (and Palm, via firmware) to enable simultaneous data and voice. It may come later this year, assuming the Pre takes off. Or it may not come at all.
Simultaneous data and voice is currently not possible over cellular. Simultaneous voice (over cellular) and data (over Pre's WiFi) is possible.
With Sprint's current configuration, if a user is engaged in cellular data transfer and a call comes in and is accepted, the data session is paused. After the call is ended or rejected, the data session is resumed. A preference in current Sprint smartphones allows the data session to remain uninterrupted and for incoming calls to route straight to voicemail. If a user is conducting a data session over WiFi, a call can be placed normally.
Based on videos of the Pre emulator, a user can interact with the Pre and its applications during a call (HTML5 aids).
It's a bummer, but recall that the iPhone (EDGE) did not allow simultaneous data and voice.
I believe this is a minor issue, at best.