When it's using Skype! While it's not the standalone Skype client a lot of people are hoping turns up for the Wi-Fi enabled N91, Useful Skype Phone allows you to use a small number of Series 60 handsets (the 6630, 6680 and 6681) as a Skype handset. The secret sauce is in a bluetooth based server on your PC, so you'll still need to be close to your desktop, but it could be a lot more comfortable than some dodgy music speakers and flakey microphone.
When is a Series 60 Phone Not A Phone?
So essentially it turns your phone into a bluetooth headset that isn't actually a headset? Great.
It's not series 60 (indeed it doesn't even involve a cellphone!) but my skype solution is my old cordless phone connected to this device: http://worldcall.brinkster.net/pcphoneline/skype/vta1000.htm
Beats bluetooth for range, anyway!
Hmm, I just use my Bluetooth headset for Skype. I guess this is for people that have Bluetooth on their computers, but no Bluetooth headset.