ID incrementing for multiple users - please help

[eluser]phester[/eluser]
I'm trying to create an invoicing application (similar to many out there) that would users to issue invoices to their customers.

This is what I'd like to have:
Each user signs up for an account. And can start to issue invoices. Invoice IDs should start at 1000 and auto increment sequentially for each of their invoices from there.

My dilemma however, is how can I have each user start at 1000 and have these IDs sequential for all THEIR invoices only? The only way I can think that is to have separate databases for each user, but I dont like this idea! I'm sure there's a better way out there.

Another way to do it is to have an ID integer field and manually increment it so that the IDs for their clients are sequential.

Any help is greatly appreciated. I hope you get what im describing.