There are some very tight limits on memory usage. You’re given approximately 64 MB of space to work with (about half of what’s available.) If you go beyond that, Springboard shuts you down unceremoniously. That combined with the fact that there isn’t any swap space where unused objects can go to rest, makes for some design decisions that you haven’t had to consider in the desktop environment.
Also, a new program called ZIPhone can jailbreak any version of the IPhone, right out of the box.
And Flash for the IPhone is coming...