In most Euro countries I am surrounded by both 900 and 1800 MHz and never have had a problem no matter how I hold the iphone 4. I have even tried asking others to hold it left and right and I have yet to see any negative result. It is pretty easy to conclude from that, and from the lab tests linked in this thread, that the network DOES matter.

I'm not saying there isn't an antenna issue. But in all the countries I have been to that antenna issue isn't bad enough to exhibit any problems.

I'm planning to visit the US in a few weeks. I will give it try there to see if results are different.