It's true, Nokia E60 and E61 phones do indeed have SIP clients - I've made them work with our Cisco CallManager Express setup in the office, though currently by having them register with an Asterisk server rather than directly with CCME. I don't have much to add to the existing googleable body of knowledge yet (apart from maybe pointing out that I had to force the codec to g711ulaw on the router's dial-peer for the SIP trunk to Asterisk, otherwise it seemed to want to use g729 or something), but I'll see how things go.

So why is it useful? Well, the idea of it's that when you're in the office (or at home, if you're inclined to setup a copy of Asterisk) your phone can dial out using the office phone line, which is probably cheaper. There's a lot of people out on the net saying "the SIP client doesn't work!" but that's because they're trying to register with a SIP server off on the internet, and the Nokia client doesn't yet handle NAT traversal. NAT/STUN support is, apparently, coming in a future firmware upgrade, though.

[update: I now have the Nokia phone registering directly with Cisco CallManager Express.]