Im afraid your wrong there 500, its not Notch its Tobias.
There's a person you should follow on twitter,
Tobias(Hes the guy in charge of Minecraft.net and therefore the login servers.) He basically was saying it was bottle necked then said the problem lies with merged accounts. Not his fault but he has to pick up the broken pieces.
Oh and before people say its craftbukkit its not, they recently released a recommended build which should be stable. If there is lag or crashes its because of the plugins mainly, they still might not be properly updated.