I've had three flats on my cyclocross bike in the past couple of weeks, all of them at the valve stem. At first I thought it was the cheap rim strip that came with the "Vuelta XRP Cross for Disc or Rim DoubleWall, black anodized" wheels allowing contact between the stem hole and the tube / stem junction. I replaced it with quality tape but low and behold I flatted in the same way yesterday.

Basically it looks like they are getting cut right at the base where the stem joins the tube. I'm running about 35#'s in my 34mm tires with Kenda Q 27-32 tubes. When I put the tape on I checked for sharp edges but didn't notice anything out of the ordinary.. any suggestion?

It is usually either the rim (check the valve hole carefully for burrs or other sharp edges) or operator error (being too rough on the stem when inflating).

Grab one of the flatted tubes, it's basically garbage now anyways. Chop the valve stem out of the tube so you have about a square inch of rubber with the valve stem in the middle. Rip the stem out and you will be left with a square patch with a little circular hole. Place this square patch over your new tube's valve stem and it will protect against future valve stem damage.

Is the cut on the thickened area next to the stem, or right next to the thickened area? I've noticed a big difference in the size of the thickened area between different brands of tube. Recently I replaced a REI/Kenda tube with a Michelin Airstop tube. The Michelin's thickened area was twice the size of the REI/Kenda's. The REI/Kenda tube failed right next to the thickened area, but I've had no issues with the Michelin tube. I didn't find any rim strip problems or rim burrs either.

Good suggestions guys, I'm going to recheck for burrs, double up the area with a spent tube cut-out and possibly change brands. the cut is about an 8th of an inch below the stem / rubber joint.