Which browsers DISABLE 3rd party cookies by default?

This is the second time i hear someone referring to "that the new browsers block 3rd party cookies by default".

Now..can please someone clarify or confirm this?

I have a product which RELIES on 3rd party cookies..but i have several Windows OS and browsers (XP, Vista, W7), IE7, IE8, FF and NO browser blocks 3rd party cookies by default.

Is there a percentage of how many users browse and actually have 3rd party cookies disabled?

G.