I'm not sure if this is a support or feature request. I have a very small test site with just a few modules installed. One of the modules (l10n_update) is installed with the very latest dev version to fix a bug in the approved version.
I wanted to run pm-update to upgrade from D7.19 to D7.20. However, I didn't particularly want to upgrade the installed modules and especially I didn't want to change the version of l10n_update.
Although I have found a --no-core parameter which allows you to upgrade only the modules, I have not found the reverse (a --core command for example). Nor does there appear to be a parameter to exclude modules (or a dialogue to update modules selectively), which I think would surely be useful.