Ameritrade's rules are that you have to have 2,000 in your account before you can apply to have it converted to a margin account.
To day trade, you have to have 25,000 in your account. If they mark you as a "pattern day trader" (basically means not holding overnight) they threaten that your account will be shut down until you supply the 25,000 for your account.
I believe this is because of NASD trading rules instituted in 2001 or so.
Always screwing the little guy, eh? :)