My limited research has shown me that most Blogger use of $\LaTeX$ relies on a small piece of javascript embedded somewhere on the page that calls a larger piece of javascript (possibly located on another server) that in turn invokes some other hosted program that actually does the rendering (possibly located on yet another server): lots of potential points of failure. Another unfortunate aspect of this method is that it doesn't allow the rendered math to show up in readers - you have to look at the page itself to have the script run.
If you have lost your math rendering you can learn a little about these things by reading the replacemath.js docs and the mathtex docs.
For now, I am using the following bit of script to render the math:
<script src="http://mathcache.s3.amazonaws.com/replacemath.js" type="text/javascript"></script>
<script type="text/javascript">
replaceMath( document.body );
</script>
Place this somewhere on your page, and it will invoke the replaceMath function that is defined in 'replacemath.js' - see the documentation about this at the link mentioned above.
Although the rendering isn't doing quite as nice a job as the watchmath version did, it's not too bad. I'll look for something better when I have the time...
PS: This blog post provides a good overview of the LaTeX rendering options that are out there.
