In theory, 3GPP standards allows simultaneous HSDPA and circuit switched voice calls on a DCH. The standard even describes how the terminal can signal the maximum speed to the network it supports on a DCH while an HSDPA session is ongoing. In practice, however, this does not seem to be implemented yet.
While testing such a scenario with a Motorola V3xx in practice I noticed that an incoming circuit switched voice call makes the radio network interrupt the HSDPA session, put the data connection on a dedicated channel (DCH) and then opens an additional dedicated channel for the voice call. During the voice call data transmission is possible with 64 kbit/s in one network (Tim, Italy) and 128 kbit/s in another (SFR, Paris). In a third network I tested this scenario the incoming call was not delivered at all. As this network has not officially launched HSDPA yet, I grant them anonymity.
After the voice call is over I would have expected that the networks put the data connection back onto HSDPA. I was quite surprised that this was not the case. Instead, the DCH was upgraded to 384 kbit/s. It would only go back into HSDPA mode once the data transfer is interrupted. While this is not a problem for web surfing, where many interruptions occur, this behavior is very undesired for file downloads. Here, the file download will continue but never be put back on a faster HSDPA connection.
The picture on the left shows the result of the test run. At first the download speed is around 75 kbyte/s. Then a voice call is started. This shows on the graph with the short interruption of the data traffic followed by a download speed of around 7 kbyte/s (64 kbit/s bearer). After the voice call is over the DCH speed increased to around 40 kbyte/s (384 kbits/s). The second part of the graph shows a repetition of the test (to make sure I was not dreaming).
Looks like it is still early days for HSDPA. While switching from HSDPA to DCH during a voice call could be both a network or a mobile implementation limitation, not switching back to HSDPA after the call very much seems like a network limitation to me.