It doesn't look like the blackberry client is going to be a real VoIP application. Here is part of the description from the press release at
Skype Coming to BlackBerry Smartphones in May - About Skype
"The application works wherever your mobile phone works, without needing a Wi-Fi connection. It uses local air time and a mobile Internet connection to sign in to Skype, update your contact list and presence and to send/receive calls or messages. As a result, you must have both a calling plan and a data plan to use the lite version of Skype."
"You will be charged by your mobile operator for local air time and data usage based on the type of calling service and data plan you have purchased."
"however national calling rates apply from your mobile operator due to the use of local air time"
That's too bad. I was hoping that this was going to be a real VoIP client.