It appears that, in infinite wisdom, Google have a security feature that can block an application from accessing or using your google account. I can see how this might be a problem for Google’s users, in particular their GTalk and Gmail users. In my case it was Pidgin having an issue with the Jabber service (which is technically part of GTalk). I found the solution after a little digging. I was surprised at how old the issue was and how long this feature has existed!
To unlock the account and get your application online, use Google’s Captcha page here.