Apple Change Quietly Makes iPhone, iPad Into Web Phones
wired.com
wired.com
If Apple will tweak the notification feature so that you can let certain apps play a tone over the headset when you get a notice, then you can just pull out your iPad, see who called or IM'ed and call them the next time you have WiFi. If it's really important, just go ahead and call them back over 3G.
To place this call you're going to need a method of input that fits in your pocket, allows you to select recipients and screen incoming calls in addition to sending and receiving the audio. Essentially, a iPad lite, also known as an iPhone.
Edit: As the iPad is based on the iPhone's OS, it is already capable of handling voice-dial inputs, you just might have to jailbreak it to access it:
http://www.apple.com/iphone/how-to/index.html#phone.voice-di...
However, personally I found voice commands easier than using the iPhone(1) for dialing while distracted. The lack of tactile response in the iPhone means I can't dial without paying complete attention to the device. I have similar problems with the gel USB keyboards and other devices you can get. I have had similar problems with regular 'key dial' cell phones when they lack proper tactile feel (slippery plastic vs rubber buttons with a clear centre mark on the 5 key).
(1) The iPhone is presently the only fully touchscreen phone that I've been interested in enough to try. The Pre had my attention . . . until I found out it was arriving on Bell (monopoly placed in the majority of Canada due to previously being national, and still price outrageously with very poor service and customer service) here in Canada.
Seriously, you'd replace your iPhone with an iPad and just carry it around in you backpack? Get over the single-device-to-rule-them-all philosophy. We're moving toward purpose built devices that do one, or a few things well. Your iPhone is most definitely the best device for telephony, text messaging and audio playback; just as the iPad is for web browsing and video playback.