I haven't posted that I got in on my Facebook just because I'm not ready to let my work place know that I'll be leaving in the summer. But I don't think it's bragging or conceited at all to post that you got in.
It is ok to call - I've called a few of my schools during the process and most were friendly. Of course, you don't want to ask frivolous questions or call too soon after applying, once you have confirmation that they've received all your application materials. 5 weeks doesn't seem too long for the adcomm to make a decision though - some departments take 2-3 months to process all the materials and for the adcomm to start making decisions. So perhaps you could wait until the middle of this month to contact them. In the OP's case - the adcomm seems to have made some decisions already and the OP hasn't heard anything yet about their application.
Either emailing or calling is fine if you really can't wait - what you can do is explain that you're an applicant and ask if decisions have been made and when you can expect to hear back from them.
Congrats on your UMass aacceptance! I know that fear of being the only person in jeans, but being in Computer Science myself, my guess is that there'd be quite a few people in jeans. It would seem to me that slacks are the safest way to go, but perhaps you can ask someone at UMass? Maybe a grad student there or someone from the department?
If you haven't signed anything then it definitely is possible to change your mind and walk away. Without knowing the details of the organization you verbally committed to, or the nature of your field, I can't say how much it will hurt you to back out of your verbal commitment. I guess it comes down to what you want to do after grad school - would you end up working in the same sphere as this organization? My advice is that you go with what you think is best for you. You won't be doing yourself or the organization any favors by working in a job for a couple years that you're not happy in.
The two posts you have up are really good and insightful reads. Are people evaluating applicants as much even if it's a recruiting visit (i.e. you've already been admitted?) It was a little eye opening to read that admissions committees solicit feedback about applicants from students and faculty at interview events.