No, you can do it more than one time. However, you must wait until the following year to make a Roth IRA conversion of the funds that had been recharacterized.
A retirement account to which up to $4,000 (or $5,000 if you are 50 or over) may be contributed for 2007, but deductions for the contribution are restricted if you are covered by a company retirement plan. Earnings accumulate tax free.