I used a router with a 1/2" shanked straight flute cutter to mortise out a series of housings for stair treads in a set of stringers with great results. The stringer were 2" thick Red Oak. I went in from both sides to avoid blow out on either face while cuting out 2x10" mortises. Not real deep compared to some bigger timber mortises but the same idea.
I did note that I had to go in steps of only about 1/2" of depth at a time. Atleast in oak, you would need a very stout router to take a bigger bite.
I don't think they are really designed for such rigorous work and you may find that the tool won't last long if used extensively for carving out deep mortises. Adding a shank extension would add a lot of extra side load on the bearings and motor. Loads it was not designed for. Like Mark, I don't much care for the high pitch scream of the router myself...
Shaun
