There were only a handful of models made to be "Pop-offs" in Hot Wheels history. The first one I know of was the Mini Cooper from the 2000 First Editions. They are a neat idea. They do not have the standard rivets holding every component together. Instead they are clipped together and the body can be taken off of the chassis.
And now, it seems Mattel has decided to remove the "Pop-off" ability of these models. I have 2 examples from this year where the clips have been replaced by standard rivets. I have to say I am surprised that this feature has been taken away; and disappointed too.
The following pictures show the 2010 Race World Earth Scion XB with clear green body (Right). Beside that is a previous variation, the 2005 First Editions X-Raycers version of the same model (Left).
Here is the previous model, popped off - pretty cool.
Something different with the green one...
Old one on the left, current one on the right... Hmm, no clip - no way to pop it off, its just a normal car now. :(
You can see the clip on the red one, missing on the newest version.
The next set of photos shows the 2010 Hot Wheels Performance Ground FX in yellow. Beside that is a previous variation of the same model.
What it looks like under the skin
The new one (yellow) with no clip
My disappointment confirmed - no clip.