Vaseline prevents hot-spots showing up.....

Is there some science behind it? Useful for the bowlers though. Waqar or Akram could never swing or reverse swing without vaseline in their pockets.

one of the commentators on skysports mentioned it - cant remember which one - but it was while Sri Lanka were batting - not accusing - he just mentioned about batsmen possibly doing it

The manufacturers of hotspot deny it would make any difference. This maybe true for most edges but when the ball just brushes the edge it may be different.
Brushing a greasy surface must reduce the friction between the bat and ball and consequently reduce the transfer of heat.

I also heard it on sky, I think hopeforthebest is right, it may only help in a small fraction of edges. Not sure what's in the rules regarding it, but the sooner they can get snicko in the better (in my opinion).
I understand it takes a few minutes to do, but so does the third umpire watching 100 hotspot replays!

Snicko takes about 10-15 minutes to sync the sound from the stump mic to the video feed. They both on different systems. Also surely the point of DRS is to remove the shockers from the game? Hotspot and proper clear replays can do that with ease.

yeah its been mentioned, I think in an article by jimmy anderson.