an idea someone gave me that enables tank steering but also ensures both wheels turn at exactly the same speed when running in a straight line. i've only been able to find examples in lego and without speed controllers but can't help thinking it would be useful in a paddle tug....
http://www.youtube.com/watch?feature=endscreen&NR=1&v=ttD5nVDHBiY

I was looking at Lego Technic gearboxes on the interweb thingy and came across this.

http://bookos.org/book/1510502

The books has a load of designs for technics drives and such.  The download is slow but is free.