Yes. I have a N800 tablet, and it worked fine. It sends and receives calls and video calls, full duplex using its integrated camera, regardless of the other endpoint being SCCP, H.323 or SIP (this is, however, due to the Cisco CM "translating" calls between different protocols).

The usual SIP settings were enough, registrar server, user, password, etc. Works over GPRS/Edge, 3G and over WiFi. QoS is not mandatory for it to work in WiFi, but very nice to have in an environment with many WiFi devices. Since an Access Point works like a hub because it uses a shared medium, there may be problems in high traffic wireless networks due to difference in data flows. It even transmits the presence status to the CM, so other users know that I am using the line or not. Due to some bugs on the SIP stack (Sofia), the calls don't work over VPN, unless you do some hacking on the tablet or put the call server in a DMZ.

The SIP client in Symbian E and N series works equally well. I also have used the Nokia ICC Cisco client, which is a SCCP client. SCCP, being Cisco's propietary protocol gives some extra features too (Do Not Disturb, Pick Up, Forwarding). The calls work over VPN as well. No video calls in Symbian, though.

Neither Maemo or Symbian currently support the handover of calls between GPRS/3G and WiFi, however AFAIK such feature is in the roadmap for Symbian devices.

I am a Cisco certified specialist, and I work in a systems integrator, we have deployed Tablets and Symbian phones in many customers with different environments, so feel free to ask anything else you want to know. My focus is on Cisco tech, though.