First off, would it be spiritually dishonest to keep my conversion private from friends and family, possibly even my former church (Presbyterian USA)? I'm in college so the only issue would be going to services at home over Christmas, Easter, and summer breaks. I haven't really figured out how I'm going to handle this. One aspect to consider is that my church is incredibly secular and liberal (I didn't know what the Trinity meant until I studied it independently last year), and of course anyone can receive Communion. I'll probably discuss it with my parish priest when I begin attending the ROCOR church close to campus this fall, but I'd also appreciate any advice.
In an ideal world you would be able to tell them right away and they'd be completely understanding.
This isn't an ideal world. I know when I converted I was afraid of how my mom would react. While she was clearly hurt by the fact that I was disgarding Anglicanism (which runs deep in my family, my grandfather having been a minister), she respected it. In all honesty if they truly buy into what a liberal Church teaches, then they'll be fine, if confused, over your decision.
Basically don't keep it from them if you don't have to, but also find a good time to tell them. You'll have to eventually but obviously some times are better than others. If that time doesn't happen while you're still in school, then tell them after.
Second, I still number a few atheists among my friends from my high school years of religious complacence and liberal Presbyterian upbringing. One of my oldest friends has unfortunately turned quite militant and belligerent in his beliefs, often mocking or criticizing Christianity. I've tried to slowly cut off the friendship but found it quite difficult because he lives in the same dorm. How should I react when he makes fun of Christianity? Argumentation? Silence? Utter belligerence? So far I've been keeping quiet to avoid confrontation and scorn but it makes me a little uneasy. How sensitive should Christians be with atheists who poke fun at the faith?
When I was in post-secondary I had an atheist friend, he would often make jokes about Christians, usually I would counter with jokes about atheists (although I admit I sometimes started it). It really depends on the nature of the relationship. If he's making hurtful comments about Christianity then ask him not to, and make clear that the friendship cannot survive such hostility.