Hi,
The subpack has two distinct 3.5mm jacks. The one closest to the edge is
line out. The other is line in. The little opening below the headphones
is for some sort of remote that I've never seen and hadn't heard about
till just now.
The USB on the go is indeed covered by a plastic bit that can be felt by
feeling with a firm pressure down from the remote opening. It is simply
put into sleep mode in Rockbox thus far.

-- 
Brandon Hicks
Email: hicks.brandon 2 at gmail dot com
MSN messenger: hicks.brandon2_at_gmail.com
lj: http://seether12.livejournal.com
skype: reyuth
SIP: 827802_at_fwd.pulver.com
My show on http://kool360.com runs Wednesdays from 7 to 11 pm eastern.