Sooner or later, mesh data will have to be sent between systems. It seems reasonable to presume that the format of that data could be negotiated between the systems, but one or two standard "baseline" formats must be required for interoperability to be real.
My thinking is that you need at least two formats: one for "static scenery," which supports things like mark-up for entity navigation, collision acceleration, on-demand paged downloading/loading, etc, and one for "dynamic objects," which means anything that animates or moves. Attempts in the past have been made to unify the two, but every successful, shipping virtual world actually splits these two.
For terrain, Forterra has taken steps in developing an open source, open specification format for the paged terrain use case; for more information see http://www.interopworld.com/members/ptf
For object meshes, however, there exist sufficient existing file formats. The two front-runner formats in my mind are X3D and COLLADA. Personally, I prefer COLLADA, because it has more robust support in authoring tools, and it does not over-extend itself in the level of specification. There may be others — MD5 and FLT come to mind.
I'd like to hear what experience you all have with different file formats that may fit the bill, and the pros/cons you've found with those file formats.