Before I ran to the store to get some fabric to sew together for a DIY pouch, I decided to do a super-simple hack that has ended up being my long-term solution. This has lasted me over 250 miles so far without a problem.
All you need beyond the Nike+ kit is a knife and a safety pin. I don't really know how much a safety pin costs, but I'd guess 5 cents.
If you're not familiar with the Nike+ system, you can check it out here:
Step 2: Cut your tongue
Step 4: Pin it up
Cool, that's it! If the sensor is able to slide all the way down the inside of the tongue you want to be careful to push the sensor up to the top as you lace up, but aside from that you're ready to go.
I've been using this technique for over 6 months and more than 250 miles so far and it's been pretty accurate. After calibrating the sensor at a track my records have been off by no more than 1-2% on my runs. Your results may vary.