CDMA networks (Verizon & Sprint) are unable to support simultaneous data and talk. GSM networks AT&T) can and do support it. As far as 'multitasking,' it will be as good if not better on iOS 5.
Yes, it can do simultaneous voice and data on wifi only.
You can also send/receive sms while on the phone.
AT&T's iPhone will be the only use carrier to be able to complete this over their network. An unlocked iPhone on T-mobile USA would only allow for EDGE data which is not capable (at least the type that T-mobile uses).