I'm seeing the same thing with an external USB CD/DVD drive (Philips USB 2.0 SPD3100CC, 16X DVD +/- RW).
FreeBSD 6.2-RC1 acts the same way, so it's not a pfsense issue.
There is a FreeBSD PR related to this, kern/85257.
http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/85257
Hopefully it'll get fixed in FreeBSD eventually…