Check out MP3Trimmer
Check out MP3 Trimmer:
http://deepniner.net/mp3trimmer/
http://deepniner.net/mp3trimmer/download/mp3trimmer.hqx
It will do
exactly what you're looking for as well as a bunch of other incredibly handy things. I don't think there's anything else quite like it, because it edits the MP3 file
directly, instead of requiring a convert to AIFF/WAV and then back to MP3 (which might work OK for a podcast, but not so well for any sort of music if you want to preserve whatever fidelity might have been spared by the MP3 encoding process).
It's seriously one of the best $10.95 purchases I've ever made, and the developer offers free upgrades and steadily improves the program (v2.5 was just released with some really nice updates). This program does what it does (editing, trimming, combining, repairing, dividing MP3s) and it does it really well.
I've enclosed some screenshots - one of the main program window (where you can sample an MP3 file to be trimmed/split/etc. (forgot about fade-ins/ous) and the "Join MP3 Files Option from the Tools Menu... just drag & drop the files you want to join, put them in the correct order in the list, Choose "select target folder," give the new file the name of your choice, and off it goes, joining the MP3 files together without messy audio conversions & re-encoding.
Oh, and I'm not affiliated with the developer in any way, just an extremely satisfied customer (I had a crazy long workflow on a weekly project that involved 4.2GB-8.4GB of AIFF files converted form MP3 recordings for editing and then re-conversion back to MP3 - this program saves not only 4+ hours of computer processing time, but allows me to work with the MP3 files (and not the huge AIFF files) directly, making it much faster and efficient (I can load the entire 8 hour mp3 recording as one file, repair if necessary, adjust in/out points and fades, and split it up according to purpose - 15 minute segments for iPod listening, 70 min segments of my choosing for burning to CD, etc.)
Feel free to ask if you have any specific questions...