Thanks, swingtime! That's just the type of response I was looking for. I don't want to make this a 'chance me' thread, but here are my stats:
Hook: Triple-URM--White, Black, Dominican, Native American (not registered)
GPA: 3.87 uw (including the 2 Bs), 4.6 w
ACT: 35 (36 R, 36 E, 34 S, 34 M, 11 W)
SAT IIs: 770 USH, 790 Lit
Rank: 50/850 (rank is unweighted, though); top 6%
APs: USH, Lang, & Gov this year, 5's on them. Stats, Lit, Euro, and Macro/Micro next year
Course load: Very history-heavy and all academic electives. Only B's have been in science classes
School type: Large public competitive, highly ranked
Extracurriculars: Student Representative to the Board of Education (only student on Board of Ed, representing entire student body of 3200 kids), Student Council, Tau Gamma (community service club, 200 + hours), Vegetarian Club (includes lots of activism), Students for Peace and Justice (includes lots of activism), Volleyball & Club Volleyball, freshman mentor, tutoring elementary kids
Awards:
National Spanish Honors Society
NCTE Achievements in English Writing (national)
Bryn Mawr College Book Award
National Achievement Scholar (probably finalist or winner)
National Hispanic Recognition Program (probably finalist or winner)
Either National Merit Commended or Semi-finalist; I'm on the border for my state's cut off score
Summers: All 3 summers Appalachia Service Project (160 hours), 1 summer au-pair in Odder, Denmark & traveling Europe by myself
Prospective Majors: History, Poli-Sci, Women's/Gender Studies
Recs: Should be excellent, from history/gov't & english teachers
All that said, I think it's fair to say that the 2 B's don't make, as you said, a dramatic difference on my application. Thank you for putting things in perspective

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I'd love to hear other thoughts, but at this point my gut (with the help of swingtime's advice) is telling me to apply ED without an explanation letter. Does anyone else think differently?