“I confess I always thought this part of the constitution defective, though not dangerous; and that it ought to be particularly attended to whenever Congress should go into the consideration of amendments.” — James Madison, 1789
In the comments, Michelle asks an interesting question: Are they increasing the total from 435?
The D.C. legislation is still just a bill in the Senate but, as passed in the House, there would be two permanent new House seats; one for D.C. and one more. Utah gets that second seat because it is currently the next state in line to add a seat based on the apportionment formula. After the 2010 census, apportionment would be according to a recalculated formula.
I’m guessing (at this point) that they are adding two seats to keep the number odd (for votes), though that makes little sense. More likely it is a political sap to get Republican support for the bill.
NewMexiKen’s preferred solution for the District of Columbia would be to return the city of Washington to Maryland, treating certain parts of the city as a federal reservation (like a military base or national park). While not without issues, this seems the most reasonable solution to me. (The Virginia part of the original D.C. was returned to Virginia in 1847.)
Historical asides. The first of the original 12 amendments (ten of which are known as the Bill of Rights) was about apportionment. It was never ratified. The very first presidential veto ever, by Washington in 1792, concerned the apportionment of the House. The House has been fixed at 435 since the admission of New Mexico and Arizona in 1912 (except temporarily at 437 when Alaska and Hawaii first became states). The Constitution says “The number of Representatives shall not exceed one for every thirty thousand…,” so constitutionally we could have as many as 10,000 Congress-persons.