Unless you are saving lives (a doctor) or protecting the country (in defense), or maybe a few other edge cases, the perception that you need to get back to a person in a few hours is probably more on your end than on the recipient's.
I have my funky cool blackberry - but I leave it on "Phone only", and, because I rarely ever answer it, hardly anyone ever phones me. I also have no problem ignoring it if it rings and it's not a call I actually need to take - I know it'll go to voicemail. The key is not letting the phone dominate your life. It's really there so I can phone people when I need to, or so I can allow someone to reach me when I need them to - and nothing else.