Not quite - If they have placed empty (or filled) frames for images and text boxes; essentially containers for you to fill and work with you can scale them all proportioanlly to each other. But, if there are blue Guides or a document grid, you will not be able to scale these with the former mentioned frames.
With all layers unlocked - Select All (ctrl + a)
With all selected, Group (ctrl + g)
Scale as desired. The caveat is that you cannot select guides and objects at the same time. If you select nothing, than select all will select the guides. I am not sure if they can be scaled.
Last item of note is that, from the Layout menu, you can change the margins you desired to work within.
If your ruler guides are touching your frames on the top, bottom, left and right sides then you can resize those objects and re-create new guides from all selected objects via the script called AddGuides.
you dont have to group the objects, as Daniel mentioned, which will place all the objects into one layer and may incur undesirable changes... select all and press E (free transform) then scale. This will keep the layer structure.
In CS5.x you can scale by simply selecting all objects and scale them that way. In earlier versions you needed the Free Transform tool or you had to group them first.
ah, hahaha... tx Bob, its become such a habit over the years that I never even thought to try it without the E...