Others with filing deadlines have come and gone. FEC.gov (Acrobat PDF)
Thanks...good research.
It someone ran the gauntlet on these (NJ, NY, NB, MN, OR, PA, SD, UT, CA, DE, IN, KY) it seems they would have enough delegates to cause serious convention difficulties.
But Santorum will presumably win PA (?) and Romney presumably UT...so really the biggest states remaining are NJ, NJ and CA.
Not sure about the math on that, but, all in all, it looks unlikely that someone can enter at this time and make a serious (rather than a vanity, or a spoiler) run for the GOP nomination.