LP portage # cat package.unmask 
media-video/spca5xx ~amd64
LP portage # emerge -av spca5xx

These are the packages that would be merged, in order:

Calculating dependencies -
!!! All ebuilds that could satisfy "spca5xx" have been masked.
!!! One of the following masked packages is required to complete your request:
- media-video/spca5xx-20060501-r2 (masked by: package.mask, ~amd64 keyword)
# Mike Doty <kingtaco@gentoo.org> (24 Mar 2007)
# Sorting out media-video/{spca5xx,gspca{,v1}} bug 159176

- media-video/spca5xx-20060402 (masked by: package.mask, ~amd64 keyword)
- media-video/spca5xx-20060501 (masked by: package.mask, ~amd64 keyword)
- media-video/spca5xx-20060501-r1 (masked by: package.mask, ~amd64 keyword)
- media-video/spca5xx-20060301 (masked by: package.mask)

For more information, see MASKED PACKAGES section in the emerge man page or 
refer to the Gentoo Handbook.