Check out this link http://www.patwarner.com/router_table.html

Pat Warner has written some good stuff on routers, jigs and router tables. Some of the points he makes with router tables are at odds with this discussion but I feel they provide another viewpoint.

He advocates the use of MDF because of its precise flatness, cost and ease in replacing. He tends to advise against laminating the table unless you have the skill in achieving an extremely even glue coverage as any variation in glue thickness will transmit bumps and hollows to the upper surface reducing the quality of your work.

In one of his books Pat used 3/4 inch MDF but I see in his website article above he has refined this to 5/8 inch. He doesn't seem to bother with inserts and the like (the guy is somewhat of a minimalist) preferring to bolt his router straight on to the top. He does pay attention to the stabilization of the top using the four supports (table frame) and two beams.

The Americans seem to prefer the fixed-base router as opposed to the plunge router for mounting. There are a lot of arguments for and against each with the former providing ease of bit changing if the base is permanently bolted to the table. You simply drop out the motor assembly. Unfortunately they are difficult to come by in Australia but the Porter Cable I am using is adequate and its single speed limitation doesnt present any problem with the smaller diamater bits I use in the table.