Following up on Kat's question during last night's RTFM, it looks like the ignoreip option in a jail's configuration affects the filter (rather than the actions), in that the "Currently failed" and "Total failed" counters for that filter don't increment when a failure is logged for an IP that's in the range/list of the ignoreip option.
So, in short, those IP's are completely ignored, for all intents and purposes, by fail2ban.