There is nothing wrong with the antenna
First of all, if people in your office are telling you that they are having the antenna issue with a bumper case on, they are either lying or they do not understand the issue. If the Antenna is covered with rubber or plastic, or almost any other substance, with the exclusion of uncoated metal, it is almost impossible for them to experience the attenuation, that occurs with this particular antenna issue. Perhaps the building is not well covered by At&t, or it is built with such material that little cell service gets inside.
For me, I picked up my iphone4 on launch day, and never experienced any problems including antenna attenuation, and proximity sensors issues. After buying my iphone4, I immediately bought a case and screen cover for it. It is a $600 phone, and even though I got the At&t discount, I wanted it to have protection. I finally watched the utube videos and saw how to make the attenuation on my phone, I took the case off, and made it happen, put the case back on, and could not make it happen.
If for some reason you don't want to cover your $600 iphone, then I think Steve gave you the best advice possible, "Don't hold it that way"