The folks at Zagg—they make the very popular invisibleSHIELD clear sticker-screens for smartphones—just tested to see if their iPhone 4 model works to alleviate antenna issues. Spoiler: It does!
The test is clear in the video. "Packets go all over the place" when they don't cover up the antenna spot, then stop entirely when they do. Packets resume going "all over the place" once they release the spot. When the invisibleSHIELD is applied, the dude can touch the naughty area all he wants, but transmission seems to be unaffected.
You can get a side-only version of the invisibleSHIELD for just $10, which is a pretty decent solution to the antenna issue, seeing as the shield is clear and fairly unobtrusive. A full front and back solution will cost you $25. Or, you can get a vinyl one with designs on it. [Zagg]