The loglevel definitions are in the syslog(2) manpage, also apparently in kernel.h.
Sounds like your syslog or initlog or whatnot is either getting called with a low log level, or it's not sending its output to the correct files. Either way, I don't know Debian very well, so I can only hazard a guess that your boot log daemon gets called in one of your /etc/rc scripts. I know those files also sometimes pass additional parameters to the kernel... But I can't imagine I'm going to be much help at this point, so I'll shut up now.