The offer is here:
http://www.ameritrade.com/united/

There are similar ones out there for other airlines/hotels.

Ameritrade refers to the *initial* balance. So is it possible to open an account for $50000, collect the 25000 miles and then reduce the balance to $2000 (to avoid maintenance fees) and keep the account open for the required 6 month?

Has anyone received miles for this offer even though they had an existing account? If this is possible, then why not roll the existing account into the new one after the first miles have posted?