N900 voip calls over wifi work okay. I have a pretty sucky ISP (Time Warner / Roadrunner. Monopoly in my neighborhood.) and the bandwidth I get varies all over the map from less than 100KB to the advertised 1MB, but the voip calls work and don't drop. I am always starting to talk at the same time as the other person because of the jitter etc issues. Also, probably because I don't have my router set up right, for about half a second at the beginning of the call, speech is really chopped up. After that, I guess, it allocates the necessary bandwidth and it's okay.
Setup on the N900 is effortless, as people have pointed out. Enter the data, and the N900 finds the networks for you and hops on. What I haven't figured out, and I'd be glad if the parent mentioned how he did it, is how I can *stop* it from falling back to 3G. For some reason, it's always trying to route calls through T-mobile first and I have to keep selecting wifi.
My T-mobile plan is a $10/mo pay-as-you-go which, after the first year can be extended for $1/month. So, starting in July, I'll have cell service for $12 per year. I'm looking forward to it, since whole months go by without me using cell at all. Call quality on cell networks is crystal clear.
I have nothing but good things to say about Callcentric. They're 2cents/min, and ~$4 monthly for fees and 911 service. They do need a bit more savvy to hook up than Skype. Unlike Skype, though, they actually work all the time for every call. (I don't have anything to do with either company, except paying a bill for service.)