Yahoo Slurp follows Javascript links?

I was surprised by this

[rover]

I need to track clickthroughs on outgoing links, but want to make sure that I don't inadvertently hijack, or withhold PR from the external sites that I link to. So I'm using the following solution (track.cgi is the script that increments the clickthroughs for specific link ID numbers):

<script>
function clk(n) { if(document.images){ (new Image()).src="/track.cgi?ID="+n; } return true;}
</script>

Example Link:

<a href="http://www.example.com" onmousedown="return clk(123)">Example Site</a>

This is working fine, but I was surprised to see that Yahoo Slurp is following the /track.cgi?ID=n link, for the specific ID numbers of the outgoing links on a page (track.cgi also notes the host).

I thought because it is 'onmousedown' that triggers this link and it requires the ID number, that the search engine spiders wouldn't follow it.

Googlebot and others don't seem to. Does anyone know if Slurp is somehow set up to do this kind of thing?