this has been annoying me for the last hour and ive found my way around it so i fugured it would be nice to share it.
(bare with me - first thread and everything, i bet it gets locked

)
first things first - log in at
http://www.minecraft.net - then try and play the game in your launcher.
choose force update
as long as you stay logged in on your web browser it SHOULD let you login with the game launcher
(worked 70-80% of the time for me from early alpha

)
if this dosent work ( it might not , it hasnt for me today) then simply try and play in browser - it will wil lyou that you need an update , do that and you should be able to play.