Apple has, on more than one occasion, stated that they have no interest in entering the Netbook market, but are keeping an eye on it. That said, I think it's more likely they'll release a Tablet-PC kind of device indeed based on iPhone OS or another cousin of Mac OS X.
After the iPhone was introduced, its OS was officially called OS X. After the SDK was released in March of 2008, they officially renamed it iPhone OS. It is based on the same (UNIX-like) foundation as Mac OS X is, namely the open source Darwin OS. For more detailed information, check out [link]
Indeed, Apple acquired P.A. Semi some time ago and Steve Jobs already confirmed in an interview that they will be designing new SoC based on the ARM architecture for future Apple products. As many long time RISC OS users may know, Apple has been long connected with ARM processor technology. P.A. Semi was formed by the lead designer of DEC's original StrongARM design team, Daniel W. Dobberpuhl. See [link]