Yes, USCIS has come back and revoked approved I-140's.
I wouldn't say it is common but there has been some legitimate cases that were revoked.
When adjudicators go to approve I-485, they have to re-look at I-140. Pre-2004; uscis wasn't strict with ability to pay or education qualifications. Therefore, people got I-140 approved. Now when uscis goes to re-look at I-140, they wind up applying todays standards/criteria/guidelines in looking at the I-140. If according to today's standards it isn't approvable then they will issue a notice of intent to revoke. (if you search through the forums; you will see a few postings from some members whom this happened to).
The biggest danger right now; is that as companies are applying more and more I-140's; everytime they apply for a new one; it gives uscis a chance to look at the history of the company. If they determine there is too many and financials don't support the cases then they will re-look at all I-140's (this is currently happening);