I recently constructed two small layouts using two different alternative baseboards, with good results.
The first was a 2' X 3' N scale layout built on a pre-stretched art canvas, obtainable in art supply stores. Overall I was happy with the results; the canvas is lightweight and sturdy, and takes paint and glue really well. Planting trees posed a problem which I overcame by gluing some styrofoam pieces to the reverse side, providing a thicker base for the tree stems. It was also pointed out to me that over time, if the layout is hung on a wall for storage (which seems kind of natural), the canvas may stretch. I populated my layout with very lightweight scenery and structures, but I suppose time will tell.
More recently I committed heresy by building a shelf switcher using the type of styrofoam you're not supposed to use, namely the expanded/beaded white styrofoam (as opposed to the blue or pink extruded kind). The entire 1' X 3' base is two inches thick; I obtained it at a craft supply store. My secret weapon was to coat the top, bottom, and side surfaces with lightweight spackle from the hardware store. After it dried I sanded it flat. This filled in the bumpy surface of the styrofoam and added extra strength. It's also a cinch to cut grooves for running wire, you just lay in the wire then cover with more spackle. This type of baseboard has the advantage of being extremely lightweight; if you're building a small layout that may need to be shipped, I think this is a great solution.