This is an OOTB feature of user profiles in MOSS, but it's implemented
in different way then you think. If each user goes into their My Site
they are given the option to edit their profile. Expertise is one of
the available fields there, as is the ability to restrict the visibility
of the values. There isn't an easy way to see a list  of expertise, but
the values are exposed via search.

We're aware of the Expertise field in MySite and how users can fill
that field.

Our problem is that we need visibility to know how many of the
roughly 400 employees have filled in that field. It's not practical
to open the MySite of each of the 400 employees. That's why we need
to have all the inputs from this field compiled into one list  of all
employees. Any ideas on how that can be done?

There isn't anything OOTB to do this, but you can do this with a custom app
using either the object model or the User Profile Web Service. With a
little research, you should be able to use the latter combined with a data
view web part to get the information without writing any code.