House Seat for DC

Yesterday was a great day for citizens of the District of Colombia. The House passed a bill giving DC's representative a voting seat. One problem - it may not have enough votes to pass in the Senate. The way the bill would work (as I understand it) is one new seat would go to DC and one new seat would go to Utah to balance out the new Democratic vote. 437 House seats would exist until the 2010 census when the total number would go back to 435 and the seats would just be rearranged according to population like it is every 10 years. (Just FYI, Utah will get a new seat in 2010 if they don't get one with this bill).
So why all the controversy? Why is it going to be hard to pass in the Senate? Frankly, I don't know. Those who oppose the bill are arguing that the Constitution only allows for the states to have representation in Congress. Granted, that is what the Constitution says: "The House of Representatives shall be composed of Members chosen every second Year by the People of the several States, and the Electors in each State shall have the Qualifications requisite for Electors of the most numerous Branch of the State Legislature." But it's not like the Constitution hasn't been radically changed before...
"The Number of Representatives shall not exceed one for every thirty Thousand" Utah has one representative for about every 820,000 people.
"Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons." Other persons of course referring to slaves.
So why can't we amend the Constitution to let the District of Colombia have a voting seat? After all, the census estimates 550,521 live in DC as of 2005 while the same year it was estimated that Wyoming had a population of only 509,294. How is it that over 550,000 of citizens of the United States don't have a voice in Congress? Some argue that the founders of this nation didn't intend for DC to have representation because it isn't a state or part of a state (let's ignore the fact that DC wasn't established when the country was founded). But my personal belief is that is was the intention of the founders to make sure everyone had a voice in Congress. That's the whole point of a Representative Democracy. By not giving DC a voting seat we'd be going against one of the most fundamental principles this great nation was founded on.