I don't use an iPhone, but I imagine if it's like other browsers, there's some setting that says:
[x] Accept cookies from sites [ ] Accept third party cookies
When I don't check the second box, it doesn't mean "Don't accept cookies, unless condition X is true", e.g. the web site implemented a popular hack. It means really don't accept third party cookies, and I don't give a shit if the +1 button breaks.
It doesn't sound like Safari's behavior on the hidden forms is intentional. It sounds like a bug, but it's irrelevant either way. I hope Apple patches this soon and forces people to opt in to third party cookies.
I'm glad that they actually set the default to reject them (a funny contrast to the address book policy). Firefox allows third party cookies by default because I always have to turn it off when setting up the browser on a new PC.