Wikipanion Plus for iPad 1.6a
the author of this app says:
The best gets even better! Enhance Wikipedia with queue mode, designed for keeping track of entries you want to read in the future. Add entries to the queue easily while browsing, and optionally save them for faster visiting or offline browsing. Downloading entries for offline or accelerated reading happens in the background while you browse and read other pages, so you don't have to stop and wait for pages to be saved.
Requires iPhone OS 3.2 or later.