I am an intermediate Filemaker developer but have never worked with publishing databases online. I have a client that is considering going web-based so they don't have to buy as many licenses for the hard copies of Filemaker. However, the employees of this company are allowed to create and edit their own reports, tables, label layouts, etc. in layout mode Is this possible with publishing databases online? Would multiple users be allowed to enter layout mode? They will upgrade to 8.0 and Server 8.0 if they decide to go this route.

You cannot make modifications to the database structure using web publishing.

In short, layout mode is only available from a full FMP installation. Multiple users can modify at the same time, provided they don't modify the same layout of course, although there is questionable wisdom in multiple users in "developer" mode.