By staying strictly on a single message Bush left virtually nothing that could be twisted or used against him elsewhere.
As for Lehrer, I wouldn't question for a minute if he had tipped off Kerry. But as you said proving it would be difficult.
Here's a thought for an mischevious freeper though - make up a fake memo from Lehrer to Kerry with a fake attached transcript of all the questions - something along the lines of "John - I've got what you asked for. Pay careful attention to the 4th and 17th questions. I'll sugarcoat your senate record on one and I'm gonna accidentally inflate the casualty numbers on the other. Good luck tonight! - Your pal Jim"
Just remember to photoshop his signature in, make sure the th's are all superscripted, autocenter the entire document, put it in comic san serif font, run it through the copy machine a couple times, take it down to kinkos, and fax it to CBS.
You're absolutely right. The President gave few answers that were outside the "message", meaning he gave few of any substance.