In my experience US has been good with $5/10 meal vouchers during long delays. My most recent data points are:
1/3/08: Charlotte. CLT-RDU canceled, rebooked 4+ hours later, $5 meal voucher as a United Premier.
5/25/08: San Diego. SAN-PHX canceled, rebooked on SAN-LAS-PHX 2 hours later, $10 meal voucher as a United Premier.
Luckily since then things have gotten better or I'd have more stories for you