Once you understand the video velocity envelope mentioned by JohnnyRoy, it's simple. I remember tearing my hair out with Premiere before I finally understood how to do that.
I still find editing and trimming a reversed clip in Vegas to be hurrendous. One of my few complaints about this otherwise great program. Why does it slide the content when you trim one end or the other?? Did they fix this in 4.0??
Reversing a clip is very simple if you follow these rules.
Step #1, find the frame where you want the reversing to begin (i.e. the LAST frame of the clip.)
Step #2, Split the clip at that point (press "S")
Step #3, Add a Velocity envelope to the clip AFTER the split and change the velocity to whatever negative percentage is needed (such as -100% or -50%...)
Step #4, Adjust the length of the reversed clip.
Step #5, Move the reversed clip to the desired position.