There's an interesting post over at the Search Engine Roundtable Forums about the reliability of Google Analytics data versus data generated by Urchin 5.7 software. Barry Schwartz points out a 20% hike in traffic stats through Urchin when viewing data for one of his sites.

It's important to remember that different web analytics products may use various means to track visits to your site. If you opt to run two analytics packages simultaneously, be prepared for discrepancies in your reports. Google Analytics uses a script-based method to gather traffic stats, while Urchin relies primarily on your server logs. Many bots count as hits on these server logs, but don't trigger the JavaScript tracker. They'll pass undetected through Google Analytics, but clamber through your site just like a human user would, at least according to an Urchin install that relies entirely on web server logs.