I remember having been able to jailbreak my iPhone 3GS for a period of time entirely through visiting a website and letting it exploit such vulnerabilities enough to perform the task. Searching for a related article, appears to have been possible on iOS 4.0/4.0.1:

https://www.cultofmac.com/53323/jailbreakme-2-0-jailbreaks-i...

Edit: I use ‘letting’ above loosely meaning that the specific website mentioned allowed the visitor to control whether the exploit was actually executed or not.