Tony, I have the same exact problem with mine. I've already replaced my Droid about 3 times for various reasons and this 4th one now has a loose slider that is wiggly, however I'm past my 30 day period, so I'll have to live with it I suppose.
Having had 4 different Droids, I cannot believe how much each varied in terms of quality. Don't get me wrong, the phone is great in terms of functionality/speed, but damn you would expect a higher quality of design/hardware to match.
I picked up a silicone vase from the Verizon store which definately makes the phone more solid with minimal bulk. The case has a very snug fit, and take away any gap you might have between the sliding pieces. I would also imagine the silicon case provides the best protection in terms of drops and impact absorbtion.
The case does giving the slider a little more resistance, but my Droid stil does open up (come loose) in my pocket which drives me nuts. Also the silicon provides a lot of grip but does kind of make the phone hard to slip into your pocket.
All in all, a pretty good buy, but I'm still going to pick up the seido case to see if that is any better.