Well, I don't know about the Gimp, but in Photoshop, you can rasterize a vector layer by right-clicking on it and choosing "rasterize" or by going through the layers menu. Generally speaking, I don't rasterize something until I need to add effects to it. As was mentioned, it's a one-way street. Again, though, I don't know how Gimp handles imported vector work.

Now, choosing between starting in raster or vector, for me, depends on the kind of image I want to produce. For textured maps like Mennin's Hallow, raster is the way to go. I can't imagine trying to do something like that in Illustrator. For clean-looking images like that Wheel of Time map I did recently, vector is the tool of choice. Best of all, though, is a combination of both. HandsomeRob's atlas maps are done with a combination of the two--raster for the shaded relief, and vector for the linework.