As of today, anyone with an iPhone who gets a call on it (weird, I know, but it happens) has two options: Answer, or Decline. Part of the WWDC iOS announcements today for iOS 6 include a revamped Phone app that offers more ways to "answer" a call. Plus, if you are completely indisposed, you can set a Do Not Disturb sign on your iPhone.
With iOS 6 you not only get the two previous options, but also "Reply With Message" and "Remind Me Later". If you tap "Reply With Message" you get a number of options to add to a "Can't talk right now..." message you can send to the other person. You can even reply with a quick message you compose yourself if you have time.
Remind me later will use Reminders to tell you things like "Remind me when I leave" and as you take off you get nudged to call them back.
Do Not Disturb will mute notifications and won't light up the screen. It will allow calls from a whitelist, and if someone calls you more than once the call can go through.
I look forward to this feature mostly because it's a much nicer way to decline a call rather than leave someone hanging if you let the call go to voice mail.