Enforcement of items being copyrighted seems to be very random on some of these sites. As an example, if you search "laptop" on iStock the first image that comes up clearly appears to be a macbook, yet it is on there and has been downloaded something like 12,000 times.

With my personal experience the item I have had rejected the most for copyright issues is cars. Even after all branding has been removed I think the only car shots I have ever had accepted were of a classic Mustang and one of a 60s Corvette. The rejection rate was so high and the amount of editing needed for isolating and removing branding took so long that I gave up on submitting that particular subject. Istock has never accepted a car shot from me.