Is it possible there's been an upgrade of your wireless infrastructure at work? I've heard (anecdotally) that Apple products have trouble with some of the more recent Atheros-based wifi chipsets used in newer routers and access points, but work fine with Broadcom. And it's more often that 5GHz is an issue than 2.4GHz; if you have the option of selecting between the two at work, try connecting to the 2.4GHz network. This may mean you'll have to make your phone "forget" the 5GHz SSID.