Sure. There are many. In fact the sonicwall tz170 has this ability if you are acustomed to sonicwall (not my preference though).
Almost any router/AP that supports multiple SSID will do this (Cisco 881w or Checkpoint Safe@Officecomes to mind). I am not familiar with the SonicWall Soho's full feature set but the fact that it is called soho leads me to believe that it won't work well with multiple SSID's. If your SonicWall supports VLANs and routing and firewalling between them then you should just be able to buy a multi SSID AP and build your own "Guest Mode".
The easiest way to do what you want is to put a switch in front of your sonicwall. Then buy a cheap wireless router just for your guests. Then they can share the internet connection and you don't have to worry about any fancy configuration.
