Shimano mountain bike front derailleurs and road bike front derailleurs are *not* interchangeable. If you have brifters, you'll need to use a front derailleur from one of the road groups- Tiagra, Sora, 105, Ultegra or Dura-Ace.
Rear derailleurs *are* interchangeable.

You might only need to change the front der. The mountain der.s have a different cable pull.

My understanding is that the spacing between the 9 spd XT rings and the 10 spd 105 rings are different, and particular to shifters they mate to. Maybe not.

I got a smoking deal on a take-off 105 drivetrain, but the FD clamp isn't the right size for me, so I have to try the XT FD just to be absolutely sure I really have to buy another 105 fd, too (although I have little doubt this will be the case).

I'd vote for "Not". I'm successfully using a Deore 9-speed trekking crank with my Ultegra triple FD without any problems. There's a difference in chainline between mountain and road cranks, but the spacing between chainrings is either identical or close enough to work.

As davida mentions, I think the incompatibility is that (indexed) road shifters don't pull the proper amount of cable to work with a mountain FD.

Sstyorkel: As to the first - That'd be cool if it worked for me, but I'll be all 10 spd up front. Is your Ultegra triple FD 10 spd or 9?

As to the second: That's my understanding, too, but I don't have any real sense of how different the amounts of cable pulled actually are.

I use bar ends because friction mode lets me use whatever replacement derailleurs I can find on the road.

1) The difference between 9 and 10 speed front is almost too small to measure. So close as to be practically interchangeable.

2) The difference between a MTB and a road front mech is enough that it will shift pretty crummy when mismatched. Chain dropping off the inner ring, impossible to get rid of the rattles, that sort of thing.

Other than the Deore M532 crank, my bike has an Ultegra 10-speed drivetrain. I had to remove one of the spacers from the Deore crank's external cup bottom bracket in order to get the chainline to work with the Ultegra 10-speed triple FD. Since then, I've had no problems. Shifts aren't quite as crisp as with the standard Ultegra triple crank, but they're still pretty darn good...

speaking of brifters, touring bikes, and front derailleurs ... i'm thinking of putting an old set of 9 speed veloce levers on my tourer. will the campy front shifter play nice with an old xt front derailleur (bottom swing style)?

fwiw, i'm using the jtek shiftmate for the rear.

I'd never used STI before we had our tandem built. I like em. I particularly like them when climbing hills. I also like bar end shifters. I like them only slightly less and only when climbing hills. If I were touring with the tandem in the middle of nowhere, I suspect I'd switch the tandem to bar end shifters, or pack them and some spare cables. For most trips I wouldn't worry about it too much.

I also don't mind downtube shifters but prefer bar ends. I'm just now converting my downtube shifters on my 80's Miyata touring single to bar ends so I guess I like bar ends just that much more.

Recently I was pleasantly surprised when my wife chose a new touring bike (a Trek 520) - she had a completley open mind when it came to shifters - and on road trials it turned out she preferred bikes equipped with bar ends over STI. I really didn't expect that. Maybe it was the road feel more than the shifters that drew her to the bike she chose but I get the sense she really liked the shifters ... and she hadn't ridden anything but a mountain bike with trigger shifters for the past 20 years.

I'd borrow some bikes from a shop and try them both and then make up your mind.