I'm of the EXTREMELY old school in that I think a PK should be defined by the elements in the table and not by some fabricated key.

That being said, it makes more logical sense that each table has it's own sequence. For example, (If I can borrow from a previous post) you have a table of products and one of users. Using two sequences your users would be numbered 1-000 and your products would also be numbered 1-1000. If you used one sequence 1 would be a product, 2-5 would be users, 7,8,13 would be users, etc.

Would one sequence work? Sure. I think it is more difficult to understand though. Also, you would want to keep an eye on performance with alot of users requesting a sequence at the same time...