I always use foamcore for my framing, either one of two options:
1) Beinfang Acid-Free Foamboard (acid-free, buffered, etc. for most jobs)
2) Bainbridge Artcare Archival Foamboard (for my super fancy stuff)
Matboard is simply too expensive for me to use as the backing board, and unless you're using museum-grade matboard you've got buffered, acid-free, wood pulp product in there anyway.
Perhaps framah can chime in and tell us what he uses for his stuff.
Neil