I have not noticed a decline, but decided to do some googling on the subject. According to a Carolina Birds listserv in June of 2008, people are not seeing as many hummingbirds and are not sure why.
I found a blog called Divinebunbun and she too reports a general lack of hummers for the summer of 2008. She reports:
"Online "bird boards" in Missouri, Indiana, and Ohio, report the same scarcity this year. Hummingbirds appeared in Missouri as usual, around the 24th of April. But everyone's usual customers aren't coming to nectar feeders. Normal hummer activity is reported only in one remote rural area of Missouri. Nobody knows why. Speculation about the missing hummers ranges from: the Midwest's extended winter (although hummers can survive freezing temperatures); flooding (confusing the hummers as they migrated from Mexico); a natural, cyclic decline in the population; a sinister, pollution-related population decline; and, because there's plenty of flowers the hummers don't need nectar feeders."
I also found a link to the Stokes blog from 2007 when people were noticing a shortage of hummingbirds, but many are commenting this summer as well (it's worth checking the comments). What about you? Are you seeing a lack of ruby-throated hummingbirds in the East? Or any decline of hummingbird species in the West? Is this anecdotal? Or is something else going on? Is it hummingbird collapse disorder?
UPDATE: A DC Birding Blog had some interesting observations regarding the hummer reports on ebird.