Alot of people on YouTube and the GTA Forums speculating that it's the GTA V Theme - which in a way is plausible as it sounds like they kind of music Rockstar would go for.
However I think it's just the music that will be played in Trailer 2 - which should be out before Summer IMO, and I think it's far more likely that it'll be the music in Trailer 2 than the actual theme.
Also someone on YouTube edited it to play over the trailer - it actually goes really well,
check it out.
People should also bare in mind the two obvious things - that 'L.S' stands for Los Santos, and Michael Hunter was the composer of the GTA IV theme in 2008.