[SoundWrangler]

Don't use MP3 for loops. MP3 encoding will ALWAYS create some blank data at beginning & end of file (which can be seen when you import these files into Peak, etc.) & this will hiccup your loop. Your alternatives:
1) use compressed AIF format (IMA 4:1), grit your teeth & knock it to 22K (give it a little presence boost at 9-10K first; some batch conversion programs do this for you) & if necessary go mono.
2) If this is for a Flash author, give him the audio full-res (AIF or WAV)... from what I HEAR, when Flash itself does the final audio compression (although worse than dedicated audio progr, such as Peak or WaveLab-Win) the loops won't hiccup. Watch out, though, their default bit rates for audio are brutally low! (BTW, Flash can also "stream" continuous audio, so IF bandwidth permits you wouldn't necessarily have to loop at all.)
3) As John H points out QT Player et al cannot seamless loop files. Use a dedicated audio program like Peak to check the looped file. Very high ratios of compression sometimes increase my batting average with difficult background loops. Also, beware of active delays&reverbs when you bounce - if the tail of these from one loop repetition is overlapping the beginning of the next, in PT you're hearing an overlap that won't make it into the bounced file. In these cases, duplicate the Region, then create the loop based on the 2nd half of the bounced file (which has the reverb/delay "tail" overlapping its beginning) in Peak (or Sound Edit, if you have a copy of that hanging around) or in Pro Tools itself (with no FX).