Technically it's not Bethesda's engine - it's a modified version of Gamebryo. It's unreasonable to expect Obsidian to build a new engine for it. In the first place, Bethesda would never allow it, and in the second, they only gave them a year to develop the game.
There's a scan of an OXM preview over at NMA - http://www.nma-fallout.com/forum/alb...hp?pic_id=4026
A warning to you Obsidian fanboys though - prepare to vomit in your mouth a little. Whoever wrote it is obviously a Bethesda fanboy and has seemingly never played any other RPGs (judging by some of the retarded things said). Naturally, there are spoilers about the early part of the game.