In last year's "Information for New Cadets and Parents" it was listed under "DO NOT BRING" (on page 8, might be different in this year's edition):
h. A large amount of money (exceeding $100).
Your cadet candidate will need some money while traveling to WP for R-Day and to shop at the Cadet store (when allowed) during Beast. From what I remember from last year, anything over $100 will be collected on R-Day and deposited in their Cadet Account. I don't remember there being a specific list of denominations. I remember sending a few $5s, $10s and $20s with my cadet candidate.
On R-Day my son took $100 in smaller bills just like wpmom2011's daughter did. (GMTA...)
This $$ question reminds me that he was glad he took a nylon/fabric wallet to use during Beast...this type dries faster than leather. During the academic year he used a very flat leather wallet (we took the leather one to him on A-Day).