is it safe to assume, then, that there is no need to remove our AdSense ads from the affected sites?
I think it's pretty safe. Like I said - if it were just happening to you or a handful of people, that would be one thing, but since it's so widespread, the onus is really on Google at this point.
I don't know how they do it, but if it were me, I'd probably profile all the apparently invalid clicks to look for common patterns, and only after that would I start deciding what's valid and what's invalid. Which means they probably had to let it run for a little while first.
I have to think the AdSense network has taken a serious blow to its reputation with advertisers.
Advertisers are used to it. It's happened more than once. I know for all my client AdWords accounts, I get refunds in varying amounts every month. This month there just might be a little bit more is all.
Silly question, where in my account so I look for invalid clicks to see if this has affected me?
You probably won't see it until the end of the month when they do the finalized earnings. I've known Google to adjust same day and end of month, but I've never known them to adjust "a few days after" or whatever. If you have your AdSense hooked up to your Analytics, you could possible play around with some of the metrics to figure out if you were hit or not. But you likely won't see anything in your AdSense account until you get your finalized April numbers (and if it's like they did it before - they just take out the earnings, and not the clicks or impressions or pageviews)
Going forward - if you're not using custom channels, this is another good reason to set them up. When you notice unusual behavior on one or two channels, it's real easy to isolate what's going on. If I notice something like this, I might temporarily disable that ad and swap in a direct ad or a banner of my own until I determine if it's just me or everyone else is being hit too.