I'm backing up some switches and I'm wondering if this config file should be kept secure. It is in binary so I do not know what it contains.
The password is very likely somewhere in that binary blob, so yes, you should keep the backup file safe.
A while back I stumbled across http://www.happyhacking.org/Happy_Hacking/Blog/Entries/2010/3/20_HP_ProCurve_1810G_Take_2.html which has some details on the binary.
btw: the 1810G stores the base64-encoded password in your authentication cookie, so you might want to take care of that as well...