I think the result of the trial will be a win-win for republicans. The decision is due out tomorrow and I am not sure which way it will go. I think the election should be voided personally, since there were too many problems and there is no way to know who won. For an exhaustive listing of events and how things got where they are the best source is Sound Politics by Stefan Sharkansky.

If the election is voided Rossi would win in a new election – all the polls are clear on this. But if the election is not voided Gregoire faces a no win term and will be outed in the next election (not only do people not support her election, she has violated campaign pledges and raised the gasoline tax by a huge amount already).
In this scenario Rossi then runs for the Senate against Cantwell and helps the GOP add to 3+ seat pickup.

Strategically the latter option is best over all. Win a senate seat in 2006 and take the governorship next round. It is not clear Cantwell would be so easily beaten if Rossi is not the GOP candidate.