Just a bit of summary after some PMs:
1. Make sure to put bots on agent white list. googlebot and googlebot-images are obvious choices. There are some more in the plugin readme file, but if someone can post a comprehensive list here it would be appreciated. Avoid entering "Yahoo! Slurp". User agents isn't really meant to have exclamation marks and spaces, and Yahoo are being idiots here. Simply write Yahoo.
2. I'm still looking for someone to post / PM a link to an incorrect redirect. I am still thinking it is cache related, but will investigate if someone provides a link.
3. Re deindexing issues: after some PMs, no-one has been deindexed, but one person has lost some ranking. Google shouldn't see the redirect in the first place because, as far as is known, Google image bot sets local referrers (it will show having clicked on one of your internal links). But you should put the bots on the white list to be extra certain.
If google had actually spotted the redirect, they would have deindexed images pending next crawl as far as I know.
Now, we will wait and see if there are anyone else saying they've lost rankings. At least one person has also gained ranks so we are not seeing any clear picture at the moment.
If it turns out to be a problem, we will rewrite the Imaguard in a way where WP will add a encoded timestamp to all images on the site:
Test.jpg -> test.jpg?timestamp=GDHJDT
If the timestamp is missing or more than, say, 5 hour old, then Imaguard will redirect or show text-over-image.
That way local users and crawlers would have 5 hour to go from the referrer page to the image itself. That together with the current crawler whitelist and the fact that googlebot-images is supposed to be setting local referrers anyway, should prevent any problems. The downside is of course that any picture can be hotlinked for 5 hours after each crawl.
I'm not too keen on changing Imaguard in this way (not just because of the extra work involved in rewriting it) but also because it adds another layer of complexity and also allows frequent crawlers to permanent hotlink if they crawl f ex every 4 hours.
Therefore I suggest we wait a bit to see what happens.