...my guess is that its because for some reason dk-milter is inserting line breaks in the middle of the the signature itself (see below, note the \n\t several times inside the b parameter).  i'm not sure how to verify, but i'm guessing that if you check it with those \n\t sequences removed, the signature would pass (when i look at all headers in yahoo they just show up as one white space).
Jun 19 14:23:57 xxxx sendmail[18752]: m5JINpv4018752: from=root, size=28, class=0, nrcpts=0, msgid=<200806191823.m5JINpv4018752@xxxx.serverpronto.com>, relay=root@localhost
Jun 19 14:26:32 xxxx sendmail[18777]: m5JIQVT8018777: from=xxx@pottsvillealumniband.com, size=719, class=0, nrcpts=1, msgid=<200806191826.m5JIQVT8018777@xxxx.serverpronto.com>, relay=root@localhost
Jun 19 14:26:32 xxxx sm-mta[18779]: m5JIQW2G018779: from=<xxx@pottsvillealumniband.com>, size=946, class=0, nrcpts=1, msgid=<200806191826.m5JIQVT8018777@xxxx.serverpronto.com>, proto=ESMTP, daemon=Daemon0, relay=localhost []
Jun 19 14:26:32 xxxx sm-mta[18779]: m5JIQW2G018779: Milter insert (1): header: DomainKey-Signature: a=rsa-sha1; s=alum2008; d=pottsvillealumniband.com; c=nofws; q=dns;\n\th=received:date:message-id:to:subject:to:from:x-mailer;\n\tb=FwoBc3XIo/VHo9RWqzK9DihmEemj9wcw5Rta7S+swZLh369eJ2JeU2KR5Y14ireZV\n\tMT7mt3DNKcrGYG5JuFq2O7SXujUFv9AknPiHmF00lxEQbS8CgP6EQQo6dyjNuG0I5d8\n\tq3sPOiGQEzrpn8WUm8VffvtKEiR0WNdb1Nex2Qs=
Jun 19 14:26:32 xxxx sendmail[18777]: m5JIQVT8018777: to=autorespond+dk@dk.elandsys.com, ctladdr=xxx@pottsvillealumniband.com (0/0), delay=00:00:01, xdelay=00:00:00, mailer=relay, pri=30719, relay=[] [], dsn=2.0.0, stat=Sent (m5JIQW2G018779 Message accepted for delivery)
please HELP!  i've been pulling my hair out for the better part of two days over this now...