Don't think in the past that this would be too much of an issue as not that many people browsed pages over, especially, 3g. But it's increasingly the case that phones especially are being relied on as a more frequent viewing option.
More sites would need to bare this in mind during site design. I'm sure the iphone issue is as Jim says - the background color is default black and the ad-image fills the color - iphone I would suspect, limits the size of single file downloads as it's a fixed entity memory wize. So in this case, default to blue and perhaps a lower quality image would perhaps solve the situation.
However, as big as phone audiences get, the bread'n'butter is ofc @home computers - so u don't want to detract quality from that. That and maybe the ads are from an agency.
It's definitely something to think about for site designers on a whole.
btw. Safari aint showing the smiley selection on the right as I type this