I turn off Wifi often when I'm out just to save battery life. I've tried to connect to Wifi at home today and can't turn it on. When I go to Wifi in Settings, it is grayed out and says No Wifi, so I can't change it in settings. This problem has only just started today. I have been able to do this previously.
MacBook 2.2 GHz Intel Core 2 Duo, Mac OS X (10.5.1)
Did you try "rebooting" your iPhone? Press the "home" button to bring the screen to the "main" menu with all the icons, hold, then press and also hold the on/off switch at the top, keep holding both buttons til you see the Apple logo, then let go.