There are no hard and fast rules for roleplaying in the new edition, true. I think 4e does at least as much to encourage roleplaying as any previous editions do however, more than some in any case. For one thing there are several pages of guidelines in the new PH. Starting on page 18 is an entire section called 'Roleplaying', covering alignment, deities, personality (with subsections on social interactions, decision points, dire straits). Page 24 finishes it up with a discussion of mannerisms, appearance, and backround.
By comparison, 2nd edition has a single page (#9 in my 1994 copy) devoted to 'The Real Basics', then it's quickly on to an example of play, a glossary, and rolling ability scores (all by page 13).
The 3.5 PH has a bit more. There are a couple of paragraphs on 'Adventures' and 'Playing the Game' (covering things like what you need to play, dice, ability checks, attack rolls, and the combat round). This is all on page 5 (the second page after the start of the book). Chapter 6 is where to look for advice on roleplaying: Alignment, the deities, vital statistics are all covered, also, looks, personality, backround, and last but not least, customizing your character. These are things that never got covered in the 2nd edition PH at all (maybe in later supplements).
I think D&D has always assumed we know how to and will roleplay. All I really need are a few guidelines and I'm good to go. I actually prefer less rules tie-in when it comes to roleplaying. So for instance, I like the alignment-lite approach that 4e takes.