So, here's what I see. Skype has two apps. One is called "Skype Mobile", and the other is just called "Skype".
"Skype Mobile" is only available to Verizon Wireless customers. It allows free calls to Skype users using your mobile phone's voice line (not data). The advantage of this is that your voice line is likely to sound better than if they had compressed it for transmission over your 3G data connection. The other benefit of using your voice connection is that you can simultaneously surf the web using Wifi (Unlike AT&T, Verizon's network can't handle simultaneous voice & 3G internet). Skype-to-Skype calls doesn't use your plan minutes.
"Skype" (available to all 2.1+ users) only uses your Wifi data connection (in the US) to make calls. It also includes multi-user chat. The review referenced in post #1 in this thread says that this app taxed the phone and that audio quality wasn't that great. I would probably only use this app as a backup to the first one, for those extremely rare moments that I have no phone service, but do have access to Wifi. Again, this is rare, but maybe if I'm in another country on vacation/cruise, this would work to call friends/family back home (but just using a calling card might be easier).