@Alephito:
Yup, I followed the wiki, full apt-get purge, erase base files (backed up databases) but it still doesn't ask me to reregister.

@Harianm
That is an amazingly useful thing, but the problem I've got is I've ALREADY got whatsup working, but I didn't realise it's the wrong phone number!
Do you know how I can force it to act like I've NEVER used it before?

This is amazingly annoying, even after everything it still remembers all of my customised settings?
HOW IS IT STILL DOING THIS?
WHERE THE HELL IS IT SAVING THINGS?!