When the Pearl first came out in September this was discussed in detail. The problem is that the Pearl port is 2.5mm and standard headphones are 3.5mm.
At the time there were some adapters that worked, but Seidio came out with two adapters made for cell phones. [They now have some others, including a retractable model].
The lesser priced adapter is an adapter only, the higher priced adapter includes a microphone and answer call button.
I have both. They work fine and have allowed me to use my high end head and ear phones when listening to MP3 tunes or watching video. Link to SeidioBlackBerry - >> 8100 - Headsets/Audio Adaptors/FM Transmitter