Simple and gloriously weird.
I registered and that login, in IE9, has persisted nicely - to go trello.com and there I am.
But if I go to another machine and try to log in I can't (username or password not valid) same thing in a different browser. Doesn't matter if I use my email or my username.
Obvious question is whether I've remembered the password correctly? So I follow the forgotten password link and I get "member not recognised". This is true using both my email address and my username (which I can cut/paste from the instance where I am logged in).
One assumes that this is a bug - but nowhere else to ping support.