If you do a search you'll see that there should be a few threads here on yellow tint in photos. People have had that problem - I don't know whether to say "a lot" of ppl, because what is a lot? So I'll just give you my own experience - my first iPhone 4, pre-ordered and came on June 23rd, had yellowish pics but not as bad as some I've seen. It had a really bad reddish discoloration all over the photos tho, so I took it back and they replaced it right away. The second phone they gave me also had yellowish photos, but again, not as bad as some, so I let it go. What I couldn't let go was a bad battery. I brought it back and they replaced it again, which is the phone I'm currently using. It has NO yellow tint whatsoever - pics are perfect IMO, and I'd be happy to share some if you want.
Some ppl have had success exchanging phones for yellow tint, others have been told "it's just the lighting" or "a software update is coming to fix it". Could you post some pics of the yellow pics?
If it were me I'd make a genius appt and try to get it exchanged, if it's bad.