Personally I think doing things in the opposite manner is more amenable to RP. Have the player roll a skill check and THEN RP the results. If they rolled a horrible diplomacy check then they describe committing some horrible faux-pas or something.
You know, I have never thought of that. It's an interesting idea, and I suppose in keeping with normal use of skills:
e.g. player declares they are going to pick a lock = make thievery check = result of check points towards the description given by the DM.
I am just not 100% sure it would work in conversation, and might confuse players by pointing towards the outcome before the event (so to speak), I think it might stilt any conversation to a greater degree than the normal
speak then roll when asked order.
conversation start:-
DM: what are you going to say?
PC: brief description
DM: makes allowances for the context then asks for roll
PC: makes roll and is informed on the outcome by DM
PC: says what they were intending to say, but altering it within the context of their success/fail dice roll
Having written all that I am assuming I must be overcomplicating it somewhat as it is far too unwieldy.
PC: declares use of diplomacy skill, makes roll calls result
DM: gives single word response - Great, good, OK, poor, terrible etc
PC: says what they were going to say, either with the confidence that it will be well recieved or intensionally bad due to poor roll.
I suppose it might work, with the right group. I still think it is best to just let the player say their piece and respond, calling for checks as necessary. After all you never know what's going to come out of your players mouths, it could be an inspired and elequently put point, or it could be a well meaning but offensive or undiplomatic statement (whether that is what they intended or not!)
For conversation I generally work on the general level of skill, position and context of the PC speaking and only call for checks when they say something that might require it (e.g. they don't
in reality realise that what they have said was inappropriate - the check is to let me pretend the character said something subtley different which wouldn't cause offence - this is an example of where character skill seperates itself from player skill)