I would guess Puerto Rico and DC as the next two states most likely to join. DC was actually very close to becoming a state in the 70's and I think that with Democrats now controlling Congress and a majority of state legislatures it could happen in the near future. It appears as though the 110th will at least be giving DC a real representative instead of a non-voting delegate. Many top Democrats have said that a proposed bill to give DC a vote and Utah a new vote will be a high priority for the 110th Congress since it appears unlikely that the 109th will get to it this year.
Just out of curiosity, what is required for this to happen, to give an extra House seat to both DC and Utah? Would it just be a bill passed into law through the legislative process (passed by both members of Congress, and signed by the President)? No additional hoops to jump through in order to expand the size of Congress, and even award House seats to a district that is not a state?