Representative David Wu has said - intermediaries report - that he will not run for re-election in 2012, but plans to serve out the term until then. If he chooses to do that, there's almost no way to force him to do otherwise. The House can expel members, but that recourse is extremely rare. On the other hand, Wu changed his mind about a 2012 run, so he might on the idea of resignation as well.
If that happened, what would the procedure be? We find this piece of Oregon law to explain:
Filling vacancy in election or office of U.S. Representative or Senator
(1) If a vacancy in election or office of Representative in Congress or United States Senator occurs before the 61st day before the general election, the Governor shall call a special election to fill that vacancy. If a vacancy in election or office of United States Senator occurs after the 62nd day before the general election but on or before the general election, and if the term of that office is not regularly filled at that election, the Governor shall call a special election to fill the vacancy as soon as practicable after the general election.
(2) If a special election to fill the vacancy in election or office of Representative in Congress or United States Senator is called before the 80th day after the vacancy occurs, each major political party shall select its nominee for the office and certify the name of the nominee to the Secretary of State. The Secretary of State shall place the name of the nominee on the ballot.
The last such case, remember, was senatorial, when Robert Packwood (in the wake of numerous allegations of sexual harassment) resigned in the fall of 1995. Special primary and general elections were held, the latter in January 1996 and won (narrowly) by Democrat Ron Wyden, who has been in the Senate since.