They've alread said that they're not working on a new elder scrolls at the moment
Oh? Care to provide an exact quote? As far as I am aware, there is no such statement that they are not working on an Elder Scrolls title. They said (around 8 or 9 months ago) not to expect any ES news anytime soon. In which case, it entirely hinges on the definition "soon."
and with New Vegas coming out I don't think they're to worried about releasing another new game anytime soon.
You do realize that Bethesda Game Studios has no involvement with FNV, right? Bethesda Softworks (the publishing arm) is overseeing that, and Obsidian is doing the development work.
Ask yourself:
If Bethesda split into 2 teams after Oblivion's release, one to work on FO3 and one to work on Oblivion additional content, what was team number 2 doing after it finished the final Oblivion DLC?
If FO3 was released over a year ago, what has the team that hasn't been working on FO3 DLC been doing since then?
When FO3's DLC team finished Mothership Zeta, what project did they begin work on?
After all, it's not like Zenimax is going to pay the bills for BGS developers to just sit around for a minimum year-long period, depending on the team in question.
Further, Todd mentioned at Quakecon that Bethesda Game Studios is working on another title, one they are not releasing the name of.
Pete reiterated that in his BethBlog clarification post, reaffirming this hush-hush in-house developed title.
And might I mention that Todd or Pete within the past month have been making comments regarding completely redesigning their basic engine tech?
And now we turn to trademarks. Zenimax isn't holding onto too many right now. But they do have a prominent one that happens to fit the Elder Scrolls bill quite nicely. Namely, Skyrim.
And what's more, they've been steadfastly renewing this trademark (which costs money and legal preparation) at every six-month interval.
Such renewing implies that said trademark is valuable, and that Zenimax is going out of its way to prevent it from slipping out of their hands.
Such implied value further implies that Zenimax and Bethesda have intent to use said trademark.
EDIT:
And it would be well to add that releasing another Fallout title around the same time as FO:NV would be disastrous for Bethesda as a whole. Bethesda the developer would be competing with Bethesda the publisher, which is a lose-lose scenario.
So if Bethesda isn't sitting around doing nothing, if they're secretively working on another title, if FO:NV is not being developed by them (or any other of the games bearing the gamesas label, like Brink), if making FO4 would cost them money and competition with FO:NV, if they're reformatting their engine, and if they're holding onto a trademark that points to TES, that paint's a picture. No?