Here's one example of how we attach super structures to our base frame. In 2012 we did something similar to what DampRobot posted, and in 2013 our superstructure consisted of four 1/4" plates serving as a gearbox and arm mount. Not a great example for general purpose robot building.
The white superstructure frame was welded at the ends to 1x1 1/8" wall box tubing. The tubing had a 3/16 rivet hole pattern in it, as we try to do for all of our parts, so we drilled out 4 holes per side for #10 clearance. The tops were drilled out for tool clearance. The corresponding holes on the chassis were drilled out and 10-32 rivnuts were installed. Rivnuts are awesome as they let you bolt things together to material of any thickness without having to hold a nut anywhere. To install or remove the entire superstructure we just screw 8 bolts in or out.
There are a number of ways to attach things to the frame without welding them to another member. Flat gusset can do a lot if you pay attention to how these gussets are loaded. The VersaChassis 90 degree bent gusset is also a big help.