I've been doing this unix lark for a pretty long time now but I'm never afraid to learn something new. Today's lesson.. the 'yes' command.
Part of the gnu coreutils, it echos a 'y' into your script (you can alter the string to a different character or a word too). So you want to install 500 packages and it's asking you to say yes to them all? Try something like this:
yes | pkgadd -d /cdrom/Solaris_10/Product `cat pkg_list`