If you look at the graphics that folks have posted in this thread, it shows pretty clearly that HTC doesn't want you to contact the ring around the camera.
It's pretty easy to see that Mr. Hand in the Apple video was gripping both sides stronger than most people would and most likely is touching the camera ring with the finger that sits in the back.
Of course that would cause a signal drop, as that seems to be an exposed section by design.
However, there is nobody that holds their phone that way naturally.
It's verging on deceptive, but that's par for the course with the big Apple these days.
On an anecdotal basis I tested it, and where i'm sitting right now i'm getting 1 bar consistently. Testing the "Apple grip" on the Eris gives these results....
ONLY TOUCHING NON-METAL AND BOTTOM OF THE PHONE: -93 to -97dBm
MIMICKING THE APPLE GRIP WITHOUT TOUCHING THE CAMERA RING: -94 to -99dBm
MIMiCKING THE APPLE GRIP WHILE TOUCHING THE CAMERA RING: -99 to -102dBm
My conclusion is that it's certainly possible to drop signal with the Eris if you interfere with the antenna by touching the camera ring and blocking some with your finger(s). However, it's very difficult and not a natural grip. Of course, you can make it appear natural (as Apple did in the video).
That being said, a -5 to -10 dBm difference is a hell of a lot different that what people are seeing with the iPhone4 (usually in the area of -20dBm). I'm gonna say Eris FTW, and Apple is being fairly disingenuous. Of course, without a side shot of their video, who can tell for sure.
[EDIT: I actually popped my back off and realized the ring is part of the plastic cover... *headslap* Anyway, it appears it's all about finger positioning.
[EDIT TAKE TWO: I re-did the test just looking at finger positioning. If I use it the way I always use my phone, I hold it on the lower 2/3rds anyway, so consistently have -90 to -95dBm where i'm at right now. If I slide my hand up to the top 1/3rd, it goes to -103 to -105dBm. Obviously, because i'm covering the antenna. Looking at the Apple video, NOBODY is holding their phone that way.]