I read that they bought the IP outright and leased it back to Interplay, for a fee ~ A fee they lose if they don't produce an MMO by 2010.
This is partially true, but there are few additional caveats to the agreement. Here is a link to the actual contract:
http://www.sec.gov/Archives/edgar/data/1057232/000117091807000324/ex10-49.txt
Here is a summary:
-On April 4th 2007 Beth acquired from Interplay all the intellectual property from the fallout franchise for $5.75 million
-Beth agreed to licence the right to make a fallout themed MMO back to Interplay in exchange for a royalty of 12% of any net profits.
This license is non-transferrable (Interplay can't sell it to anyone else) and is forfeit (goes back to Bethesda) if any of the following occur:
1) Interplay fails to secure $30 million in "minium financing" and start development within 2 years (April 4, 2009 deadline)
2) Interplay fails to commercially launch the MMO within 4 years of starting development (option for 1 year extension with Beth's approval) - (April 4, 2013 or 2014 at the latest)
3) Interplay fails to maintain the MMO with at least 10,000 subscribers for a three month period after launch
4) Interplay goes into bankruptcy or merger (someone else buys them)
5) Interplay fails to pay the royalty.
I think Bethesda made a great deal.