So long as you don't touch the line or court inside the line before the ball has left your strings then it's not a foot fault.
I'm a big Safin fan and I've noticed and tried this as well.
I also played Varsity Basketball in H.S. , went to basketball camps etc. and moved like this to dunk.
Coaches called it a "drop step".
Safins left foot is not the only one that shuffles toward the line.
The most extreme left foot shuffle I've seen was a top 100 guy named Raemon Sluiter at the US Open a couple years ago, I think he made 2nd round.
He moved both feet so much it looked like a running start but didn't get called.
It made me think of the possibilities...
maybe someday in the future we will be taking Michael Jordan slam dunk from the foul line running jumping serves.
The problem with all this extra movement is that the more things you try to do before hitting the ball, the more difficult timing in all those extra motions become. This is why, in general I teach my students that simpler is usually better.
However as the average serve speed increases for top players and the rest try to keep up, I wouldn't be suprised if we see more complicated techniques evolve.
www.advancedtennis.com has some great slow motion videos of Safin for reasonable donations.