Expression to allow keyframing of RATE/SPEED of rotation.

Hi everyone!
First time posting on creative cow today. I asked this question already in the main forum, but realized this may be a better place to post this... here goes:


This one's got me and my colleagues stymied.
I'm trying to create an expression (keyframable using a slider control) that will animate the speed of rotation on an object.
The actual rotation value of the object is not as important to me as the RATE of rotation.
I started something using time multiplied by a slider but my quandary is that when I set my slider to zero, I don't want my rotation to go back to zero.
I want it to stop rotating, to hold at that value.
I tried creating some sort of identity switch using a second slider but got nowhere. Something tells me I'm on the wrong path.

Any help would be much appreciated!

Thanks!
-Gabe