Ok.. well the mtgox support team were pretty quick to respond - so that's good.
They say they saw 5 attempts..
Our system automatically bans IPs after 10 failed login attempts for 24 hours. Unfortunately, we can not raise these limits manually since it is set for all accounts within our system. If you can not remember your password, we recommend you to generate a password recovery email. We apologize for any inconvenience caused.
I guess this is reasonable (though now that I've gone for a long password - I'm pretty likely to get it wrong a few times!)
They've been kind enough to reset it for me.