I know that over here in the UK it's not allowed, anyone guilty of "product placement" can get massive fines from the ITC. I don't know how it works with foreign made TV shows, i.e. stuff from the USA like the Sopranos, I'm assuming there isn't much they can do about it.
If you ever see Knowing Me, Knowing You, with Alan Partridge, (Particularly Knowing Me, Knowing Yule) you'll learn about the potential consequences of product placement