Here's what was happening. The Google code looks something like this:
Code:
<script src="google.com/conversion.js"></script>
<noscript>
[img]google.com/?script=0[/img]
</noscript>
This is supposed to display a visible "google sitemeter" widget when the JS works, and if JS is disabled, display an invisible 1-pixel image that ticks a counter.
When I pasted the code in, the first time or first few times this page was accessed everything worked fine. Then, the next day, invariably the JS code would no longer be loaded, and neither would the 1-pixel image.
So after trying several different things (moving Google's comment tags, moving the JS declarations inside Yahoo's own JS on that page, etc.) which had the same effect (works first day, doesn't work after that) I figured I'd punt. I reasoned Yahoo may be blocking loading third-party javascript, certainly a reasonable security concern, and I couldn't get help from their tech support.
I removed the <noscript> tag so the 1-pixel image would display even if JS were enabled. That works. Now at least I'm getting conversion stats.
My concern now is that I may not be getting accurate or full conversion statistics using the image.
Does anyone know more about whether their 1-pixel hack tallies accurate stats? Has anyone had better luck with AdWords and Yahoo Store?