I did it a year back when wifi was just introduced onboard, even though the brochure said voip was blocked, skype worked just fine ... Now I did so with NO seat mate, in domestic First class and ONLY did so to see if it was blocked. I WOULD NOT do it AGAIN and will PROTEST till the cows come home ANY effort to allow telecommunication inflight! I am a techno -geek and did it just to see IF it could be done. I have heard from friends that certain ports are blocked and they have not been able to connect to voip, mostly via gogoinflight.
On a side note I was on a DL DC9-50 flt to ATL two weeks ago that had gogoinflight aboard.