Broadbeam's Uni-Fi Networking platform lets clients seamlessly roam between Wi-Fi and cellular networks: The offering supports all the flavors of wireless, including 802.11 a, b, and g as well as GSM, GPRS, CDMA, 1xRTT, Nextel's iDEN, EV-DO and EDGE. I've seen plenty of announcements from companies that say they offer this capability but few leap to mind who have actually deployed it for real customers. London Ambulance service uses this for ambulance dispatch in a 620-square-mile radius. Each ambulance is equipped with gear that can operate over two different GPRS/GSM networks and Wi-Fi.
Another user is CSX Transportation, the rail network operator in the United States. Train conductors use an application to report on their work process throughout the day. They can use the local area network, 1xRTT, or GPRS, or data can be stored and forwarded later if the conductor is out of range of any network.
I wonder if Calypso, which recently received a patent for its technology that enables roaming between networks, has or will go after Broadbeam. I've been surprised that more companies don't want to talk on the record about the Calypso patent, which could be narrow enough to allow other ways of enabling such roaming. But I suppose it's a legal issue and companies may want to let their lawyers handle it.