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.
The Pop-offs were different, they had to be specially detailed throughout the interior because when the body is removed, everything is visible. I also liked the idea that every time they came out with a new variation, you had an increasing number of combinations of tops and bottoms to fit together.
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 I have 2 examples from this year where the clips have been replaced by standard rivets.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. :(
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.