I have an environment with multiple iMacs (OS X 10.7.4). Every iMac has an administrative account and several user accounts. These accounts are the same across all the machines. To browse the web, they go through a proxy server. The intention is for all the iMacs to behave in the same way.
On some machines, all accounts are able to browse the web normally. On others, the administrator account can browse normally, but the user accounts cannot. They are unable to even contact any servers via http. However they are able to ping the same locations as the administrator account, such as within the network and to the proxy server. Their network settings under System Preferences > Network > Ethernet are identical, including all proxy settings. The parental control and user account settings on both systems are identical. The settings for their web browsers (Safari and Firefox) are all identical.
I'm not going to rule it out as a network issue just yet, but some indication of where to look would help, or if there is some obscure OS X setting which could be preventing web browsing for specific users.