So how do you reply to the argument that "people certainly like iOS devices that are locked-down, often citing the absence of malware and other non-conforming apps"?

That I want to see that real user of iDevices that sober will say the phrase "noncomforming apps" ... or that lack of malware will be anywhere near the top 5 reasons of liking them. And path behavior was one expected of malware. And it was approved by Apple with bells and whistles.

Malware is a solved problem already since Vista shipped even on windows. I haven't seen one in ages on the maybe 20-30 friend pcs that I am the involuntary "IT" guy.

People like iDevices. Period. But one of the engines of innovation is being able to use something for purpose other than the one envisioned by its creator. It creates positive feedback loop. Which include running software that somebody might not approve. Also unlocked does not mean defenseless. Sandboxing and isolation are good things, so is the vetting of the apps in the store.

I think that the HTC model currently is best - device comes locked, with official unlock available that voids part of the warranty. If I want security - I got it. But if i want to use 300 of them mounted on the ceilings of the clubs uploading realtime feeds to show the hottest places in town right now - I can use them for that too.

I'm still seeing malware on Vista and 7 on home machines where the users are STILL all local admins and click "OK"/"Yes" to every "should this be allowed to run?" prompt because years of popup messages have conditioned the average user to just say "whatever" and click the continue button.

Even without administrative privileges, your malware can still do lots of interesting things: full access to the user's files, settings and browser history, full internet access, automatic launch on startup.