11 Apr 2008 edit
document.write and tell the site owner to just include it in their page where the ad or content should appear.
If these services want people to stop complaining that their sites are loading slowly as a result of adding these tags (and perhaps looking at least for solutions that minimize the number of scripts that need to be loaded), it's time that they expose an API that will allow sites to include their
script tag at the end of the page's HTML, where it belongs. Then sites could call a method to insert the ad wherever it's needed, once the content has finished loading.
I grant that this would require a bit more code from the ad providers (and that their ads would load a little bit later than they do now), but the improvement in user experience would be tremendous, relative to waiting as much as a second or two for the ad — and thus the rest of the page — to load.
Update: If you're looking for a fix to this problem, also check out John Resig's document.write() rewrite.