He tells us the whole story in a very long post at the frontpage of muxtape .
Go and read it!!

- I love music. I believe that for people who love music, the desire to share it is innate and crucial for music itself. When we find a song we love, we beckon our friends over to the turntable, we loan them the CD, we turn up the car stereo, we put it on a mixtape. We do this because music makes us feel and we want someone else to feel it, too. -

Well this article by Listening Post is a must read!!!

As the RIAA shuts down unlicensed mixtape services such as Muxtape,
the RIAA’s main constituents — Sony Music, Universal Music Group and
Warner Music Group, all equity partners in MySpace Music — have
approved the same sort of user-generated playlists on MySpace,
according to Fortune.
[read it in full @ Listening Post ]

Muxtape will be unavailable for a brief period while they sort out a problem with the RIAA. Muxtape says: – No artists or labels have complained. The site is not closed indefinitely. Stay tuned. -

have you ever made a mixtape??
a properly cutandpasted for parties?
or a sentimental for your girl?
or none? ever??
never mind…
the muxtape is the easiest way to do it, really. Just upload your songs (max. 12 mp3s, not bigger than 10Mb) and you are done. I just made one with my chill mashups to test the system and its works well. You have to register yourselves, and upload the songs one by one. When the uploads are done, you can edit the  artist names and song titles and the order of the songs. The only visual customization possibility is to change the color of the header in your muxtape profile, but its fine as long as the main thing is the music in the muxtape. You can listen the muxtapes made by other users so its FUN.
For about 3 weeks till its gets shut down, as someone commented in the boingboing, but till then its FUNFUNFUN ;)