Interesting thread, here's my $0.02.
My dog's a german shorthaired pointer / field lab mix, very athletic high energy dog. He can easily do 10K at my jogging pace, as I trained to this distance and he was the ideal training partner. Now that we're rolling as a family more, my wife on her Elektra cruiser pulls the 1 yr old in his Burley, and Laswell runs alongside me on my fixie. Some lessons learned (some more painfully than others):
- The Springer device some people use is quite unwieldy due to a) being difficult to hook and unhook if the dog's gotta "go" b) making the width of the bike an extra six inches (spring-arm + dog) out to the right -- wider than the zig-zag gates on our local MUP and difficult to manuver (That K9-Cruiser from adogsbestfriend.com looks better than the springer but I haven't used this personally)
- I just use a short 4-foot leash now. 6 feet is too long - my only spill so far was when I was using a 6-foot leash and Laswell bolted for a squirrel, across my front wheel. I endoed but he was unharmed and the squirrel got away. 4 ft keeps him just in the right place.
- DONT PUT YOUR HAND THROUGH THE LOOP at the end of the leash. Just hold it against the handlebar so you can just open your hand and drop it if need be. This keeps you upright and keeps the dog from getting throttled if something happens suddenly.
- Work with your dog -- s/he should absolutely be trained to "stop" on command, and its also nice if you have voice commands for turning, speeding up, slowing down, etc. I basically keep up a running patter of commands and encouragement while we're riding to keep him engaged and under control.
- As others have said, keep an eye on pads, hydration, and exhaustion.
It's good fun, safe if you're safe about it, and great exercise for athletic breeds.