I have the opportunity to buy a relatively generic domain name (there doesn't appear to be any registered trademark involved) where the .org has been in active use for 8+ years. Would it be considered OK practice to build out the .com?
One other issue to worry about is that you will have to size up the .org. If it seems like a big organization, they may, at a later date, claim that they had an unregistered trademark since they have been around for 8 plus years as you state, especially if you become big and a thorn in their side. If it is a mom and pop then they probably wouldn't bother going through the expense.
But if you go through with it, you also may want to consider getting a trademark registered yourself because as time passes it would become harder and harder to get it expunged and the potential for trademark problems will be minimized.
As others pointed out, if you're planning to use it for something totally unrelated that's one thing, if you're buying the .com to target the .org's product or service you may (or not) get away with it but it's a really lame thing to do ...
I do pro-bono work for an area organization (using a .org) which has been around for about 8-10 years, with a DMOZ listing and some nice search rankings to show for it. A while back I discovered a parked page on the .com with keywords relating to the organization's very "niche" business.
I doubt the .com owner is getting much out of it - by now the group is heavily branded on the .org and anyone typing it in is more likely to click their BACK button than on those links, but still (and I'm saying this as someone who doesn't have a problem with GENERIC parked pages) its obnoxious.
As I recall, one of the group's members is an intellectual property lawyer. One of these days I may have a little chat with her about it ...