Comverse demonstrated a product that lets Wi-Fi users send SMS messages to cell phones: This may seem like a no-brainer but SMS isn't an IP-based system—so you can't send an actual SMS from any computer. This blurb, however, makes it sound like wireless operators may be migrating toward IP SMS systems.
Here is an actual press release from Comverse on the capability. The idea is to let Wi-Fi-enabled PDA users send and receive SMS with cell phone users.
It will be interesting to see if this is the way people will want to message between devices or if instant message clients, which are already becoming available on cell phones, will handle messaging between cell phones and devices like PDAs.