Meg Gardiner is a newbie to the US scene is, but has been published for years in her adopted home overseas. She made her stateside debut this year with Dirty Secrets Club and reprints of her original series that began with China Lake. Stephen King raved about her in EW and now you should all find out why.
Another debut (debut debut, not just here) this year that stood out for me was Kitty Sewell's Ice Trap. In the book, a doctor receives a letter from a girl claiming to be his father. Problem is, he can't remember ever having been with the mother in the first place. It's a one-sitting-read, trust me. Look for Kitty's next book in sprin '09.
Of course, my fave in the genre is the incredible Mo Hayder. I have to take every opportunity I can to recommend her books because they are so utterly fantastic. Not for the faint of heart, though. She's extremely graphic, something that has earned her some criticism but also praise in the book world. Devil of Nanking (aka Tokyo) and Pig Island are stand alones. Birdman, Treatment, and Ritual can be read alone or as a series (in that order).
Finally, not a debut author but a new one on my radar is Libby Fischer Hellman. She has an existing series, but debuted a new heroine this year with Easy Innocence, a book that will really open your eyes to the world of today's urban teen (I sincerely hope that most of the country is not living like Hellman's girls).
So check these out for yourself, if you haven't read them yet, and buy them for your thriller readers. They'll love you for it!