Here is the link to the state run DCNR trails.
http://4wheeldrive.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=4wheeldrive&cdn=autos&tm=15&gps=65_1214_792_568&f=00&su=p284.9.336.ip_p706.12.336.ip_&tt=2&bt=0&bts=0&zu=http%3A//www.dcnr.state.pa.us/forestry/atv/registration.aspx
Basically, summer trails are for summer use only and the summer & winter trails run from the Friday starting the Memorial day weekend to the last full weekend in September. So they are closed from October until the end of the anterless deer season set by the PA game commision, which this past season I think ended on January 9, 2010.
Right now you can ride at the Adams Cumberland County trail (dark blue logo) referred to as Michaux State Park until 3/31/10 and then it is closed until 5/28/10. It is a rather rocky/boulder trail system with just a handfull of nice smooth trails. You don't have to have a utility or 4x4 there but the IRS will make it nice for you guys!
I rode it on my 07 Wolverine 450 4x4 and never used 4wd and I would certainly go back again but a IRS would of made it awesome.
I also hear that Bloody Skillet (dark green logo) is a really great trail system and there now are talks or plans in place to connect it to SSRT (
http://ssrt.org/lodging.htm ) which is not state run and is really affordable $15 for the first person in a household and you can buy the membership at certain local stores when you get up there but it cost like an extra $2 or $3, check the website for more details.
Hope that helps you out.
Dan