Halogenation

halogenation of ketones?? unheard of this. [:O]

http://en.wikipedia.org/wiki/Haloform_reaction
Haloform reaction is halogenation in basic medium.

yaa, i know it of course, but you didn't mention the base..(or simply I was silly enough).
This i only I could deduce: glycial acetic acid is anhdrous form of acetic acid. Throughout haloform process, the water evolved should removed simultaneously because it can drive back the reaction. Now taking in knowledge that glycial acetic acid exists in dimer state, it hold the capability to dehydrate the water as soon it is evolved in reactin through it's crystal lattice. That's why it is preferred in reaction.
Disclaimer: I'm not so sure of this reasoning, this is what I think is possible in this reaction. Searching about it a bit more.

@ ankur : in the question im talking about halogenation in acidic medium..not haloform rxn.

I think he's referring to halogenation of ketones to gem-dihalides.