How to create random, but unique IDs?

[eluser]Steven Ross[/eluser]
[quote author="huzzel" date="1273709694"]a purely numeric ID with no more than 8 digits couldnt be a uniqe id.[/quote]Why not? Unique within our system. We have a few thousand items for these IDs, the number grows over time, but will likely never exceed 100,000. Eight digits gives me 100 million IDs to choose from randomly. More than enough, isn't it?

And those IDs get exposed to users, so I'd like to avoid huge clunky hex numbers like what sha1 produces.