Skype's mobile client for Windows Mobile has been updated to 2.2, it now is compatible with Windows Mobile 6.1. GSM Arena put it through its paces and found it to work well over WiFi, over 3G not so much. Otherwise you're looking at a pretty full-featured client, with support for calls, Skype-out for calling regular numbers, Skype's text-chat, etc.
One downside that GSM Arena notes is that, unlike when making normal calls, the screen stays on during a call and is therefore likely to get “cheek taps” that might accidentally end your call. 'Course, that's not a problem for WMExperts' readers, because y'all know about the clever software that just turns your screen off with the push of an assigned button.