Hi FF that's right, the eagles are the paid for currency.
You can grind to most aircraft and get XP but it's slower without eagles. There's a few game areas that you need a premium account to access - again paid for with eagles.
Cool, pretty much standard stuff for MMOs. I'm pretty sure I purchased Wings of Prey via a YuPlay account way back, so I'll dig back and see if I can find the email I used. A peek at the WoT forums seems to indicate that they might be opening up the invites for a few more North American people etc.
Looking forward to it now