This was a great thread. I've been riding with Spd's and mountain bike shoes. What's the difference, right? I've had intermittent "hot foot" on longer, harder rides and I was going to switch systems with my new bike, but confusion ensued. I put super feet insoles in for the last long ride, and they didn't help. Now I'm going to go shoe shooping 
Darcy, the "intermittent" might be the heat. Interesting.
"Well-behaved women seldom make history." --Laurel Thatcher Ulrich
'09 Trek WSD 2.1 with a Brooks B-68 saddle
'11 Trek WSD Madone 5.2 with Brooks B-17