This is for archive and Google indexing purposes, since the fix was so deucedly hard to find:

Problem - I've upgraded Movable Type from 2.6x to 3.x and now all the comments that were previously approved are in Pending status, and they cannot be manually approved, ie. nothing happens when the link is clicked.

Solution - Delete the lib/ directory from your MT install (ie. at /home/username/public_html/mt/lib) and reupload it from the MT upgrade files.

Apparently some of these files are a bit tetchy about being overwritten, which causes the conflict that turns the comments to all pending. More specific details are available in this MT Forum post (scroll down to the last entry, please).

It is also recommended that BEFORE you begin the upgrade, you remove any MT Blacklist installation, because the version you're running with MT 2.6x is NOT compatible with any of the 3.x releases. Appropriate versions can be downloaded from the MT-Blacklist site.

From the Hostmatters forums, a bit of wisdom on circumventing nasty MT comment spammers:

The key is that spammers have automated scripts that look for Moveable Type blog sites and they then post to our comments using a direct call to the "mt-comments.cgi" script. If you installed Moveable Type into the default directory (/mt) then they know exactly where the script is and how to call it.

The solution is simple: rename the script to some odd name (ex. qwerty.cgi) and edit your mt.cfg to point to the renamed CGI script. Look for the line that is commented out and reads "# CommentScript mt-comments.cgi". Uncomment the line and change the name of the script to the new name. You need to rebuild the site before it takes effect. Users will not be able to post comments while you are doing this but the entire process only takes a few minutes.

I've been getting slammed with around 30 comment pr0n spams a day lately, trying this method in 3, 2, 1...

It's a fact, AOL Instant Messenger is about as good a product as AOL itself. Which is to say, not at all. DeadAIM is a nice little program that enhances AIM's *cough functionality and gives you more control over the little bugger.

So you've just paid to have your corporate site redesigned but not one of the overpaid monkeys in your IT department will demean themselves by even learning what HTML stands for, and the secretary's computer skills extend to pressing Place Bid in an effort to gain the latest sateen monstrosity destined to grace her already overcrowded (and extraordinarily creepy) boudoir. What is a bottom-line-conscious company to do?

Contribute is a nice new tool from Macromedia that allows the most thumbfingered member of your staff to update a website without requiring a server reboot and complete restore from backup. It's WYSIWYG interface resembles a web browser, allowing knuckleheads to drop in new pictures, edit text, etc. etc., all without borking the underlying code. It can even handle *gasp* hand-coded sites, such as those turned out by, yes, Sekimori Design.

But don't just take my word for it, take the tour...