...wicked
The easiest way is to bounce the mix out and do the edit on the mix file. Sound Forge can whip this up quick and easy, I think you can also do this in Reaper by automating the master playback rate.
A month or two ago Craig Anderton had a Sound on Sound article on how to fake varispeed treatments in SONAR that I also seem to recall mentioned this in some way.
You don't need Sound Forge or Reaper. Like true analog tape variable speed, speeding up shortens duration and raises pitch, while slowing down lengthens duration and lowers pitch and Sonar can do this. The most common variable speed tape effect I did
by far was speeding up the final mix by 1 or 2%, which of course also raised pitch by 1 or 2%. This technique easily accomplishes this function, as well as the "Chipmunk effect" and a bunch of other cool stuff.
The speed change can be up to four times faster or slower, and you will not hear digital artifacts. In other words, it’s just like using tape except that with this technique, the speed change is not continuously variable. The thread referenced by scook has that and other "varispeed" techniques.
1. Open the clip in the Loop Construction window.
2. From the Clip drop-down menu, enable Stretch On/Off.
3. Move the Threshold slider all the way to the left (0%) so
all the markers disappear. This is very important.
4. The two right-most fields adjust semitones and cents respectively. Do
not enable the Pitch button! That will just confuse things. Cents will adjust +/-49 cents which should be enough. If not, for example if you need to make the pitch 70 cents sharp, set semitones to 1 and cents to -30. (For Chipmunk effects, set semitones to +12

.)
5. Render the clip, and it will reflect the pitch/speed changes you made.