I'm getting the same problem: from around the time I upgraded to WP2.7, archive links on my Cipher Mysteries site now silently redirect to the homepage. This is true for the default archive widget, the collapsible archive widget, the drop-down archive widget, and also if you physically type in the archive address (i.e. http://www.domain.com/2008/06 etc)
What is frustrating is that even though it silently redirects to the homepage (oh yes, and it's not a redirection plugin error either, I checked), WP2.7 fails to emit an error message to the error log about what is going on, so there's no obvious way of diagnosing this helpfully.
Right now, my best guess is that this might well prove to be a PHP 4-related bug that has silently crept into the WP2.7 codebase along the way, but which nobody thought to test against. Has WP2.7's archive behaviour been tested on sites running PHP4?
I noindex the archive pages anyway, so this is not a terrible loss: but it's irritating all the same, and it would be very nice to know what to do about it.
Thanks, ....Nick Pelling.... // Cipher Mysteries