I upgraded from 3.2 to the latest version -- 3.35 at the time.

All spam filtering seem to stop at that exact moment. I have Akismet, Spam Lookup -- Lookup, and Spam Lookup -- Link. All the settings look normal, but spam filtering is almost non-existent. I do get a lot flagged as spam, but hundreds of spam comments per hour are still getting through, and it's killing me.

Is there anything else I should look at? Is there any known issue with spam filtering when you upgrade?

This is unclear. Your title suggests the spam blocks aren't working but your actual text basically says theyt are but that stuff is getting through. These are significantly different situations, so which is it?

Were you always getting hundreds of spam messages per hour? That seems excessive. Are you sure you're not just getting flooded at this particular moment, which would explain the difference?

Sorry, it is filtering but at greatly reduced effectiveness. Several hundred spams an hour is accurate -- I delete several thousand per day.

What it seems like is that the upgrade killed Akismet, but I have no proof of that. All the settings are unchanged. How can I prove or disapprove that Akismet is working?

Well, first of all, which Akismet plugin are you using? The Nonplus one or the official one?
With the official one at least: go in to examine a specific comment, and underneath you'll see a heading to says "Final Feedback Rating." That section will give you some basic information about why a comment got blocked. If Akismet's working, you should see something along the line of "Akismet -1.0 Akismet says spam" among other things. Check a few.

I suppose an unpleasant if easy way to find out if it's working is to turn off all your spam plugins except for Akismet and see what happens...

Yeah, that proves it. There's is nothing from Akismet since the upgrade, but there is from before the upgrade. The upgrade apparently killed it.

Thanks -- I'll poke around based on this intel.

Turned out the upgrade cleared the WordPress.com API key, which, in turn, caused Akismet to stop working.

Thanks for your help on this.