iPhone Multitasking is not multitasking in the true sense. Its restrictive in many ways as you would expect from Apple. I don't know if the multi tasking enabled after jail breaking is any different. In any case read more about it here.
Ars reviews iOS 4: what's new, notable, and what needs work
While on it Android Multi-tasking experience is not currently flawless either. With each process gobbling up a insane 20~40Mb of RAM, Multi-tasking is not practical unless you have a fairly powerful phone loaded with RAM. I am not all impressed with the Multi-tasking experience in Android 2.1 (Eclair) on my Galaxy S. Hoping Froyo would improve this with HIMEM enabled. I guess the only major mobile Platform with optimized multi-tasking is Symbian and even that has its limits due to the minimalistic CPU's used by Nokia.
As for Flash, iDevices are not going to get flash support officially and hacks are the only way around it.
On the Android front, Android 2.2 (Froyo) is slated to have full flash experience, but even some 2.1 devices currently have the flash lite support. I guess HTC Sense enables Flash support on HTC Android devices (Not sure whether all HTC Android devices have flash support). Samsung Galaxy S has it too in its browser, but it comes disabled by default. I have it enabled in my Galaxy S and it works well.
A phone is no replacement for a net book even for browsing, but if you want a phone for that purpose, you would be better off going for an Android Phone like Galaxy S. If you are willing to sacrifice Flash support just to get an iPhone, then better go for the iPhone 4. Trading Flash support for browsing on a 480 x 320 screen would be really sad.
An iPad may make a better choice, but I guess the reason you want to ditch a Netbook is because you don't want a second device around. Sacrificing the functionality of a Netbook and then spending about twice as much for an iPad and then carrying it around just for the compromised browsing experience without flash does not make much sense. However if you want a compact tablet just for browsing, then wait a bit and you have a number Android tablets all shapes and sizes flooding the market soon. You might find a cheapo 5~7" tablet suitable for your purpose.