If you haven't read it, I had a 40 barrel fit to my Colt Delta Elite: http://reg7.site90.net/?p=129
I have been using my 10mm mags in the 40 conversion with excellent results. However, the difference of in the COAL of 10mm vs 40S&W made me think that getting 40 mags would be better and allow me to keep the ammo separate. No problems yet, but safety first.

I got three at CDNN for $12.99. They are blued 8shot. I figured they were a good deal even if I just used them at the range. I have a mixed review.

Here are a couple of pics. Again, sorry for the low quality, they are from a camera phone:

MTM follower. Surprised at this design. It is not super stable, allowing a lot of tilt/pivot. One of the three mags didn't lock the slide back when empty. The mags are only a 8 shot. It is almost as if they should be 9 but couldn't get it to work.

MTM feed lips. While the short feed lips seem to work well with the 10mm, adding the spacer for 40 means that the round is pushed further forward. Simply inserting and removing the mag causes it to shift forward/back. The lips are also not very tight, the round pops up slightly--can't be good for the mag (stretching the lips) and making mag ejection hard.

MTM baseplate we are used to.

This is kind of a mixed review. One mag didn't lock back the slide manually in 3 1911's I tried. Strangely, 2 of the 3 mags did not eject clean out of my Delta Elite (they drag in the well), although the eject fine when empty so it is not actually a problem.