Red-headed Woodpecker population numbers continue to decline, but some local populations have rebounded a bit.
Here is the BBS trend graph for the Prairie Hardwood Transition Bird Conservation Region (southern WI is in this zone):
These two graphs look very similar. They point to continuing long-term decline.
Birders see a few individuals here and there, and there are some very localized increases. That provides very little real information about their population as a whole, but the expanding use of eBird, and the unfolding Wisconsin Breeding Bird Atlas II will gradually fill out this picture. The decline is occurring in many areas of their geographic range, including but not limited to Wisconsin.
Here are some articles with much more information:
Birds of North America - Red-headed Woodpecker
"The Return of the Loud Redheads"
"The Population Decline of the Red-headed Woodpecker in Wisconsin and Illinois"
WDNR - Red-headed Woodpecker
National Audubon - Red-headed Woodpecker