Today Tobias and me tested some options for determining the height of the fork on the forklifts using RFID. These options are very similar to the barcode option. After the first test we noticed when the tag was on a metallic surface the range decreased severely from 7cm to 3cm (estimated).
A quick google search found some interesting papers/advertisements confirming this. Nicely enough these result also gave some basic (and very obvious of course) tips on how to avoid and work the diminished range.
One of the tips was to get some distance between the metal and the tag. We tried it, and it worked better, but mainly because it was closer to the reader now.