What does the following apt-get response mean?

[Tom H]

On Wed, Dec 1, 2010 at 2:14 AM, John Conover <conover at rahul.net> wrote:
>
> What does the following apt-get response mean?
>
>    root at mymachine:/ 520# apt-get upgrade
>    Reading package lists... Done
>    Building dependency tree
>    Reading state information... Done
>    The following packages have been kept back:
>      linux-generic linux-headers-generic linux-image-generic
>    0 upgraded, 0 newly installed, 0 to remove and 3 not upgraded.
>
> Why won't it upgrade it?

What's the output of "apt-get --simulate dist-upgrade"?