Same setup: HTC Incredible S with latest official update (Ice Cream Sandwich), 2 weeks ago the problems started:
Skype is running, then, during an attempt to call mobile or landline phone (for which I have a subscription + some cash in my account) the Call button will grey out and would be impossible to press.
The only way to fix this temporary is to manualy log out and log in again (seems Skype logs out randomly, without informing me).
During attempt to log in again my saved password\name won't be recognised and I'll have to enter it again till the next time (in a matter of minutes sometimes)
It's a known issue I understand that Skype is aware of and looking into.
Is your connection on Wifi or on 3 mobile internet when this happens?