I am beginning to texture a new model that has several separate objects. Is it possible to combine the separate UVmaps for each object into one map that contains everything?

yes! The UV layout can be treated simply as a canvas, you can use different parts of that canvas for different objects.

Plan where you want the different parts to be and lay the UVs out for each separate object. carefully so that you won't introduce overlaps.

The fun bit comes if you want bake procedurals or export the UVs, in which case you will need to do it for each object and then paste together in your paint package.

The only question I would have is why do you want to do this? - is it that objects are very interwined and you want to continue maps over from object to object seamlessly? if this is the case you may be better off booling / joining the objects together?

or is for some other constraint?

The object(s) concerned is a lorry trailer with moveable drop sides and tailgate. As I was mapping the bed I began to wonder if the UV's could be combined onto the one map as they would all essentially share a similar texture. Just had another thought, if you were to UV a wheel and then duplicate it, would the copy be able to use the map from the original object?

It should make an exact copy..  I believe it uses the same material  (instead of making a duplicate of the material).

Yep that's correct reddog, where things may get fouled up is if you mirror the duplicated object...