Below are some discussions from the Panam-riders list on Google groups.

We visited Machu Picchu a few years ago and loved it, though it was raining all day :)

On this post on the Panam-Riders blog are some questions and answers from the Panam -riders about the subject:
http://panamriders.biketravellers.com/machu-picchu-how-to-for-biketravellers/
Add your tips there!