What model iMac is it where you are visiting? If it's an iMac with Wifi built in you can use connection sharing and share the wired internet to the Wifi on the iMac. Then the iMac will transmit the signal it's getting from the wired internet and your iPhone can pick it up like any Access point.

To enable it you open System Preferences and Sharing. Then click on Internet Sharing and select the Ethernet connection and share to WiFi.

I like the Airport Express idea also, but if that is not possible right now, this should get you by.