It seems blocking homebrew isn't the only thing that System Menu 4.2 does. According to some reports from NeoGAF and Nintendo's own tech forums, it's also causing Wiis to get bricked.

Not everybody is affected, but it doesn't just happen to HBC-enabled Wiis. Apparently the update has dodgy boot2 code that has a high chance of causing write errors. It looks like this is the cause of the brickage.

Nintendo hasn't said anything official yet, although there are complaints that some tech support forum posts about the issue are being deleted.