I'm sure it happens on the bigger more powerful websites (example IGN and Gamespot both giving GTA4 perfect 10's) but as someone who has a small working relationship with publishers regarding NintendoLife.com, WiiWareWorld.com and my personal blog get2sammyb.co.uk - no publisher has ever pressured me into giving a high score. They try to sell the product to me of course, but they don't bribe.
It does happen though. The main problem is with publishers wanted to advertise on publications and harsh scores leading to the publishers threatening to pull their advertisement money. No advertising money means no magazine so... :\
There's an interesting series of the BonusRound on it.