If nanotechnology were to be developed to aid in extending human life
on earth, I would imagine the same technology has applications elsewhere. For example, getting off this rock and aiding in survival in conditions unlike earth. Bored with earth? Fine, go somewhere else.
Besides that...
Everyone has their individual definition and vision/fantasy/whatever of immortality. Boredom, exploration, et cetera. We're looking at it from this small perspective of the present. In order for us to answer even slightly on the same page, we would need a more detailed scenerio than,
"Bam!! Humanity is immortal, deal with it!" As of now, some might see it as magic. What is magic? Art Clark says
"Technology sufficiently advanced is indistinguishable from magic." Tell someone from the early 20th century we'd be surfing around this virtual place called the internet and describe ALL the shenanigans and goings on of the entire web... let alone a bunch of geeks surfing for pr0n and arguing endlessly on a flight sim's BBS. That might be a lot to take in.
I would imagine the deveopment of nanotechnology would not reach a level so advanced as to stymie aging all at once. That would be the pinnacle of nanotechnology - all areas converge to this all encompassing process. Stages leading up to this convergent time would extend the impulse and soften the blow. It would seem only logical to blend all of hte various aspects and see what kind of gas mileage we get. Further, faster, better.
In what sort of applications would nanotechnology be of use? Pry safe to say hundreds if not thousands of applications can be thought of in less than a month's time. A few things are pretty easy... look at Deus Ex

Realistically, I wouldn't mind some sort of a tap into the optic nerve for a heads up display in your vision.
Some who are keen on the Gaia theory think the internet is a primitive nervous system for the planet and we all live in some symbiotic coexistence. Gaia, earth, is becoming aware of itself through tiny eyes (humans). Imagine a massive network which allows all humans to tap into and communicate. We could evolve past primitive spoken language... communication through other, more thorough means for better understanding. Think of telepathy through nanotechnology

I could go on and on. Basically, I don't think boredom is a good answer. Should this happen, it's not going to be like flipping a switch.