In comparison I can create an android application, put the apk on my website, and distribute to anyone who wants to download it without making any agreements with Google or putting anything on their Market. The only catch point is that user's must allow untrusted sources to be installed.
I don't consider this to be a "catch point", but a security feature -- AS LONG AS THE USER CAN DISABLE IT. Being able to disable security features is what makes them security features as opposed to vendor lock-in. I think Android is similar to Debian in this respect -- by default you can only apt-get official software, but any user can add extra sources. (It's a little different because users can run untrusted software on Debian without adding extra sources, but in spirit it is similar.) This in turn allows for competitors to Google's market, e.g. F-Droid.
If my understanding is correct, it would not be possible for iphone to do something similar.
Absolutely not. Well, again, on jailbroken devices (as I described above), there are third-party stores. Cydia is the most popular -- essentially you install Cydia on your jailbroken iPhone and then from there, you can install other software that has been introduced into the Cydia store. But again, that relies on the exploit of security vulnerabilities in the iPhone and is therefore not reliable. On a normal iPhone, there is no chance in hell that Apple would allow you to install a third-party software centre. That would break their complete stranglehold on all software and online stores on all iOS devices.