Once in a while I need to use my iPhone 4 for work related tasks that require a lot of bandwidth. When doing this I will connect to the wireless network at work, do what I need to do, and then disconnect so that any personal activity goes over 3g. Currently my choices are to either leave wifi off while at work, only turning it on when I need to do work tasks, or join the work network every time I need to do work tasks, then forget the network. Turning wifi off is annoying since I have to remember to turn it off before I get to work, and then turn it back on after I leave work. Constantly joining the network is also annoying since I need to enter my username, password, and proxy information. Is there a way to have my iPhone remember my work network so I can manually join it without entering all my information, but not connect to the work network unless I force it to? I haven’t found anything obvious on either the phone or the iPhone configuration assistant so I’m pretty sure the answer is no but it can’t hurt to ask.
As you can see from this screenshot, iOS will automatically join any available known network based on signal strength. As of right now, it is not possible to do what you are asking with iOS.