I wonder how much traffic HN is cannibalizing from more traditional tech news sources like venturebeat and techcrunch ... http://siteanalytics.compete.com/techcrunch.com+venturebeat....
A similar comparison on Google reflects little change for the others:
http://trends.google.com/websites?q=news.ycombinator.com%2Ct...
That said, it relies on GoogleToolbar/Chrome, but we could argue that it's distribution amongst visitors of those sites is going to be pretty similar.
Edit: Updated to include mashable.com
I'll probably be using Google trends in the future, thanks for the heads up.
After looking at the chart (assuming as Swannie does, that Google trends is a significantly accurate source), it seems pretty apparent that HN doesn't have enough traffic to cannibalize significant traffic from them.