I normally have audiobooks in CBR format. I just got an audiobook from a friend that is 3 8hr files. The files are VBR and when I try to play with doggcatcher and presto enabled, it shows the file as being 27hrs long. If I seek into the file doggcatcher acts as if I seeked to the end of the file.
if I disable presto, doggcatcher shows the right file length.
Does Presto properly handle VBR files? There is "metadata" embedded in the mp3 frame (not id3 tags) which gives the average bitrate you don't have to guess based on either the first audio frame's bitrate (probably not representative of the average) or by decoding the full file to see what the real average is.
Details can be found here: http://www.codeproject.com/Articles/8295/MPEG-Audio-Frame-Header#VBRHeaders