Probably better off with cordless MP3 headphones, they are quite small now, available in 512mb and are cheaper than the thumps. Afterall what about when its not sunny ? You'll be paying thru the nose just cus their Oakley. Oakley make sunglasses not MP3's, stick to manufacturers who specialise in what they do.
Look at
http://www.iwantoneofthose.com/MP3HEAVAR_ELEC.htm
OR
http://www.hi-tech.com.mt/index.pl/mubie_mp3
Mac.