Kinda. There's a difference between a bootleg remix/mash-up and a regular bootleg. A bootlegged remix is basically taking the track and mucking about with it unofficially - as in you don't have permission to use the master track so you can't easily, say, take out the vocals or something. Same deal with a mash-up, except it involves combining two different tracks, as in Sexymassive or Britney's Black Hole.
Personally, my favourite Muse remix is the Muse/NERD mash-up She Wants a Muse. Sexy as.