You don't know any windows end users? They'll install anything they find on the web.

As a former PalmTX end user, I could and did download software from a variety of sources. I know plenty of Widows end users who will install just about anything they find on the net.

If we're talking about end users that only install what is available by default on the phone, then iphone, android, Maemo are all about the same to the end user. They have an app store/default extras repository to install "approved" apps from.

If we're talking about an end user that might actually hit google and search for an app:
Iphone: can't install w/o jailbreak
Android: Can install file found
Diable: Can install file found by clicking
Fremantle: Need CLI to install

Fremantle has added an extra hurdle to the end user to install random files, ie become more "closed".

For, the record, there are plenty of deb's that can be found on the net and installed on Diablo. Many right here in our own forums. I have several pieces of software that I installed from debs not found in Extras or other repositories. Perhaps not the best idea, but it was just a click or two away to do it. Even if you don't personally install anything not found in a repository, there are plently of things out there, even now.

That option is slowly being closed off, with the push to only install from Extras, resulting in a slightly more closed ecosystem.