Safari 5.0.3 crashes
I haven't heard of any specific issues with 5.0.3 and 10.6.6. 

However, there are times when upgrades to Safari will break things like plugins or extensions. I recommend disabling all extensions and plugins via the Safari preferences. Then you can enable one plugin or extension at a time and visit a website that you know will crash your browser. If things work fine, repeat this process for each plugin or extension until you discover the one that is causing things to break. Once you find the culprit, you might have to wait until they issue a new version. 

Having said all of this, there is a very good chance your crashes are related to Flash. Flash is a known vector for Safari crashes on Mac OS X and Apple has repeteadly stated that a majority of the crash logs they receive are Flash-related. 

Even if you were not experiencing these crashes, I would still recommend getting something like Click2Flash to disable Flash on all websites--this is a great way to get rid of pesky ads. This should considerably reduce the number of crashes in Safari, assuming what Apple says is true. With Click2Flash, you can always whitelist specific websites so you can unblock their Flash content or you can unblock Flash elements on a click-by-click basis (hehce the name, Click2Flash). 

Hope this helps.