I was losing the cursor in Macrumors when a certain banner ad was showing - see this thread.
I also lost the cursor on another site (sorry, can't remember which), where the same airline ad was showing.
This happened in Safari 4 and 5, under 10.6.3 and 10.6.4. Also in Firefox IIRC. I've not seen the ad for a while and have not had the problem.
Being in the UK, I've not experienced the ad that Belly-laughs showed in his screenshot.
I'm running 10.5.8 so it's not 10.6 related. I have not had the problem since I emptied the cache (alt+cmd+E), which would have forced a reload of the Flash-banners I suspect caused the issue. Although the same ad was showing, they may have issued new problem free banners in the meantime, they have probably caused issues on other sites too.
Also, I only experienced the missing cursor issue on other sites when the Macrumors front page was open in another tab. Never seen the ad anywhere else.