This actually does not seem to hold upon checking (or at least not anymore). In Tophat 1.4.1, there is no mapping quality of 2. I've looked at a large number of BAM files and it seems that the qualities are as follows:
255 = unique mapping
3 = maps to 2 locations in the target
1 = maps to 3-4 locations
0 = maps to 5 or more locations (up to the number defined in "--prefilter-multihits").