I'm not a Yakov Smirnoff gag

[plasmid]

Hollow's my skull, think that I'm dead?

Conclusion untrue, from burrow loft

Union maintained, steel our thread

It's straight as a brick is light and soft

Recognize me? Better inspect

A fraudulent claim shall not get through

Handshake select one I reject

In Soviet Russia, teeth brush you!

[Yoruichi-san]

(...b/c my mind tends to jump to some abstract mathematical principle *whistling*)

but I'm going to go with a key or keyring.

[plainglazed]

a bear trap?

thinking there's definitely still more to parse but the above might somewhat fit. this one was just long overdue for a first guess. we'll crack it yet, plasmid

[plasmid]

Thanks for getting this one started, PG. Not a bear trap, lines 5-6 are pointing toward something else.

[plasmid]

It might be easier with a rephrasing where the clues are more clearly clues.

I have a hollow skull, but am not "dead" because of it

I have come from or been sent from some sort of "burrow"

I either am a part of or am a union kept together with steel

And that steel is not straight

The "you" in the riddle should recognize/inspect me

Any liars or impostors won't get through

I will, with a handshake, pick something to "reject"

Teeth brush you

[Aaryan]

A vending machine? Not sure about the teeth, but it's "rejecting" the thing it gives you, has the spring-like metal wires, and I guess it is pretty hollow. Not entirely sure about the rest.

[plasmid]

Not a vending machine. I can see where you're coming from with that answer; clues about maintaining a union with a thread of steel, not letting a fraudulent claim through, and (maybe) teeth brushing you could maybe be made to fit it, but I think the answer I have in mind is enough of a more natural fit that most people could recognize it as the right answer when re-reading the riddle.

[DudleyDude]

possibly a padlock? I think most of it makes sense if it's a padlock with a key but not sure if the "hollow skull" is too much of a stretch and not sure how to work in the "better inspect" line. But I think the "union maintained" section could refer to the curved shackle and the "liars or imposters" through the end could refer to the key and using the key (handshake) to release the shackle (one I reject)

[plasmid]

Not a padlock, but you're so close it hurts.

[DudleyDude]

I should try my original answer (before I talked myself into padlock). A door knob?

[Pickett]

a deadbolt...hence the "dead" references...or more generically a door lock of some sort...

Edited July 16, 2013 by Pickett

[plasmid]

Something quite close to those answers, but think of a union that's held together by a thread of steel that's curved. And the "you" in "teeth brush you" really is referring to the "you" and not the "I" of this riddle.

[plasmid]

Yoruichi has this one. Definitely on the right track with the previous guesses, just not quite hitting the actual thing that I was trying to describe.