| In case your questions haven't been answered elsewhere...
I believe Bucknell does a good job at accommodating everyone interested in a res college. It's not guaranteed, in case an overwhelming number of people apply, but I haven't heard of anyone being turned down.
To the best of my knowledge there are no mandatory commitments after your first year, but there are opportunities to stay involved. You can apply to be a JF (junior fellow -- basically a mentor for the res college) or an RF (resident fellow). Each res college also offers alumni housing in the same building or in a nearby university-owned house for sophomores who participated in that res college as a first year. It's basically like having priority housing since you wouldn't have to participate in the upperclassmen housing lottery and you live with people who were in your res college with you. I don't think there are any additional commitments with alumni housing.
Hope this is helpful. I just didn't want your question to go unanswered before you need to submit your course selection form (where you designate the foundation seminar you want that either coincides with a res college or not). Also, I didn't participate in a res college myself, but I did seriously consider it and had the same questions as you. A lot of my friends participated in res colleges, so I'm pretty familiar with it.
If you have more questions I suggest visiting the res college website, checking out the discussion boards on the Bucknell 2012 Facebook group, and/or contacting a JF for whichever res college you're interested in (info on the res college website). | 
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