According to the initial link posted at the top, it sound like she is one of the more likely candidates since several of the others are committed to other projects. I think it'll be a toss up between her and bringing Gervais back.
I've never been a fan of the Office - its style of comedy I just don't find funny - but I will start watching if Tate joins the show. I'll definitely be watching the finale, at least!
As for the play? There are ways to work around that, including NBC holding off on airing the next season until January, or December. It would be risky, but American networks have been moving away from the standard Sept-May seasons for years. Remember The X-Files started a season around American Thanksgiving at one point, I believe in order to allow one of its stars to do a movie. If NBC wants her they'll accommodate. (A great idea would be to keep Tate's character as British and have NBC film a story arc in the UK!)
Another scenario is just like the last few episodes of this season won't feature Carrell, who is to say Tate has to appear at the very start of the season?
I doubt that outside of Doctor Who or British TV fans than anyone in the US knows her...but then, very few people beyond watchers of The Daily Show knew Steve Carell when The Office started either...
That's the thing: she comes with no baggage, at least with US viewers. I remember the fear and loathing from UK viewers when she was cast as Donna because she'd created this image with them with Lauren and all that stuff. But the US is a clean slate and the most they'd remember of her was her being Donna in Doctor Who, which isn't a bad thing except for elitists who automatically dismiss anyone who's done SF.
Another aspect in her favor is her age. She's in her 40s now, which is actually a bit old for casting on US TV. Unless they're hoping to bring back the Murphy Brown vibe or attract the Sex in the City crowd. Neither of those shows appeal to me, but given there aren't many (any?) American mainstream-network comedy series featuring a female lead in her 40s right now, that could be a factor.
Alex