I am wondering the same about what's the membership level for Corporate Preferred account as well. I got the account some years back and don't have any detail any more.

From the back of the card, it stated some benifits of:

o True Redemption, including free nights without restrictions
o 2 Startpoints per elibible US dollar spent
o upgrade to a Preferred Room at check-in
o 4:00PM late check-out (excludes some resorts)
o Complimentary weekday newspaper

Seems it is very close to Gold membership, except less points per dolloar spent?

Have anyone got upgrade with Corporate Preferred account? I have not experienced any but I have only limited number of SPG stays so far.