If I’m right there was a way to use textures to emit particles on the 2.49. Is this technique available on 2.5 too?
No, you only can use Weight Groups.
Seriously 
In the new 2.55 release too ?
Nothing to do for controlling particules emmission (where it’s start and end to emit with a gradient) with texture like 2.49?
It’s on the ToDo for the final release…
Yeah, waiting for particle features to appear is like watching paint dry.