You can get a very cheap frame blend by showing previous frames at lower opacity when playing at speeds lower than 100%. So for instance, when playing at 30% speed, you would see the current frame at full opacity, the frame before that at 66%, and the frame before that at 33%. This is what FCP7 and Premiere do as well.
Personally I think it looks horrible, and I always turned it off during editing. You can get proper motion predictive frame interpolation in some editing packages these days, in which the software actually predicts how the frames in between should look based on pixel movement. But those type of effects are crazy CPU intensive.
If enough people want the cheapo blend, we could probably put it in.