Rust Tick

[Jason]

I was playing in the Scars of Mirrodin prerelease this weekend and came across this situation:

I eot, tap an artifact with Rust Tick

On my turn, I untap the tick and tap another artifact. During my opponent's untap step, he gets to untap the first artifact, correct? I'm assuming once Rust Tick becomes untapped it "forgets" which creature it tapped to begin with.

[luma]

That's right. Because Rust Tick became untapped, the first "doesn't untap" effect ended.