Okay, this is thin, but seeing how some of team TiPb are experiencing the dreaded yellow splotches on their iPhone 4, we're clinging to threads here -- forum threads in this case from Apple Insider:
Apple is using a bonding agent called Organofunctional Silane Z-6011 to bond the layers of glass. Apparently, Apple (or more likely Foxconn) is shipping these products so quickly that the evaporation process is not complete. However, after one or two days of use, especially with the screen on, will complete the evaporation process and the yellow "blotches" will disappear.
How do I know? I was involved in pitching Z-6011 to Apple.
Jeremy wonders how this explains perfectly straight yellow lines, so who knows, but it's something to consider. If you have the splotches, chances are Apple won't have replacement units for you in the next few days anyway, so keep on using it and let us know if you see an improvement...