How do you react if your opponent tells you that there's a mate in three (or whatever) and shows you that it's forced? I'm usually tempted to do this when I see that victory is inevitable, if only because the rest of the game always feels like a waste of time. I'm worried, though, that someone will take offense.

Thoughts?

I think it's a bit hammy.  It used to be very much in vogue though.

I played a guy that told me mate in 5. Yep, all forced as I was just amazed at how he did it. No offence taken. We did have a rapore built up through the message feature. 

I think it's in poor taste.  A win is a win.

That said, if the losing player notices that his opponent has mate in 3, it is a sign of respect to resign.

If you want to sound less like a prick (but still a little like a prick) then you can say,

"I believe that's mate in three." 

or,

"I think that's..."

"I could be wrong, but..."

and etc.

Also you can use the conditional moves feature to log your 'forced mate' and write "good game", "gg" or some variation.  That way you don't waste your or your opponents time.

Is almost sound like he telling you to resign (early), which is rude, if the opponent found a way to checkmate you in three moves he  should be able to use the 'conditional move' button then. If it was with a friend that ok really no harm.

It has it's merits, depending on the tone of it, which can be hard to convey over the Internet obviously.   I mean if your opponent hasn't spotted it and you point it out, at least it gives your opponenet the chance to save their time and move on to the next game "should they wish to".  In that case then maybe it is some what ok.

But if it is done purely to gloat or to demand a resignation then obviously it is a bit out order.

One thing I'll say; if you announce it, you'd better be right. Because even if the other guy just sacrifices his entire army just to make it take one extra move, it's really really bad if you announce Mate in X and it isn't.

I would say not to announce the mate.  One of the many reasons included as above would be that if your opponent feels that it is inevitable, then they will resign.  However, for the sake of the player facing the checkmate, it is often the case that if they have not seen the mate and resigned, then they are probably a less experienced player and perhaps it would prove valuable for them to play out the position and see how it works.  That's something that I recommend to all students in chess, to play out everything if you are still learning.  For those who have some experience and etiquette, they will resign.  If not, then they are probably playing the position out of spite, in which case you will have to play it out and force the mate anyways.  It just always seems to be in good taste for the sake of cordiality, manners, and for not disturbing others around you (if you are in a tournament setting) to say nothing.  If you feel that you have won the game, then rest assured and the situation will resolve itself either by your opponent learning etiquette and something about the game, or by you showing the pompous opponent that you may have who insists on playing it out until the end anyways.  It's a win-win situation.

who cares? If it's really mate in however many moves, accept it. Say it if you want, and if it offends you, you need to grow up.

Depends a bit on how it's done, if you just say "Mate in X" and expects instant resignation you do it wrong. If, as the original poster implyed, you also give the full variations or at least something like "58. Nf6 (any reply) 59.Qh7#", then it's another matter entirely. The one way I announced was against an opponent who refused to resign, and the I just wrote the lines (below) and put it in as conditionals for one of them.

cuendillar: 51...Kh5 52.Bg3 Kh6 53.Be4 Kh5 54.Bg6+ Kh6 55.Bf4#

cuendillar: 51...b3 52.h3 Kh5 53.Bf4 Kh4 54.Bg5+ Kh5 55.Bf3#

He resigned at move 54...

I'd rather have the pleasure of playing it out if i'm going to win in just a few moves. 

Do you recall that old story? Player A looks up proudly from his game and announces mate in 3 but his (stronger) opponent just returns his gaze to the board. After a minute again he looks up and declares mate in 4. His opponent does as before. A minute later he announces mate in 5. and his opponent ignores him. A is studying the board intently, seems to pause, then in a soft voice says, "I resign."

this Player A guy, is he any good?  i cant find any of his games on the net...

seriously, dont announce mate early, if its a forced variation just use the conditional moves...if its not a forced variation, ie you missed a hidden resource, then alerting your opponent may give him the awareness to look for said resource...i say keep you lips shut and let your pieces do the talking

I posted the moves to him with the very intention of showing my hand early. That way, he could 'see for himself' if there was a way to delay it. Also, the 'he might mess up' factor isn't there anymore as a last hope. Then it's up to him to decide whether to play it out or not, sometimes I agree with my opponent that the game is over and we analyse a bit before resigning/agreeing a draw.