You probably have something like "linux-image-2.6-686" installed. That is a virtual kernel package: there's no real kernel there, but it has as a dependency whatever -686 kernel is available for your Debian box (given your sources list and current package list). So, it keeps trying to install the -686 kernel for your branch of Debian - even if you have already installed a different kernel, as you have. (The -686-bigmem is different than just -686.) Some people like to have this since it will make sure to upgrade your kernel whenever an upgrade is available to a newer series. I find it to be a pain in the ass.
To see what you have installed already, do this:
aptitude search linux-image | grep ^i
Then, if you only want the -bigmem kernel, remove the "virtual -686" one (eg, linux-image-2.6-686") and anything else you don't want.
Finally, if you want to always stay in the most current series of the -bigmem kernel, install "linux-image-2.6-686-bigmem"
Man this thread seems familiar: http://forums.debian.net/viewtopic.php?t=25139