Wireless networks do seem to exist in Star Wars (at least in the expanded universe) but are avoided in many cases due to security concerns (just as in the real world).
So here are a few tips for handling wireless security from a security consultant, and some analasys on how it integrates with the Star Wars universe:
Any general use wireless (in this case by wireless we mean omni-directional transmissions like radio) technology can be hacked (with time). So most truly important communications are not done wirelessly, but are instead in a closed system with access points: wired communication, fiber optics (really just another type of wire), point to point laser communications (really the laser is just your wire), and (apparently) the hyperspace holonet. In order to hack these types of transmissions you typically need direct access to the equipment, or to intercept the transmission with similar equipment and re-transmit it. So they can still be hacked with effort, but simple remote hacking is impossible.
So it makes perfect sense that most of the hacking we see in Star Wars involves directly plugging into the network. The slicing done on Bespin and the Death Star were all examples of slicing into secure installations.
That being said, even secure installations are going to have wireless communication that can be sliced, mainly in the form of communications. Is there a remote, battery powered defense turrent out there? Probably controlled via wireles communications, and thus slicable. Battle droid controls? Wireless, slicable.
The other thing to consider is: would the information be accessable via a public network (like the web in our world). Hotel reservations, public records, bank records, and files that employees can access from home are all examples of data hackable from a public network.
So, how can you tell if something is remotely slicable? Just answer two questions: does it involve wireless transmissions or is it accessible via a public network? High security systems aren't going to be hooked up to a network or wireless. Not remotely hackable. Medium or low security systems may be (for example, if an owner can unlock or lock things from home, it is remotely hackable).
You may even have multiple layers of security within the same system. Cameras are often available remotely (so the boss can check in on things, for example) while doors are on a local system. Some doors (outer doors, for example) may be accessable wirelessly (the doors into a bank, for example) but the secure doors (the vault) may not be.
So that covers general security. There is one last thing to consider in Star Wars:
Droid uprising paranoia is a big thing. Fleets have been lost to it, wars have been fought. There has been at least 1 major droid war fought, possibly more, and many, MANY smaller incidents. So networking too heavily is discouraged, especially in military situations. Networking makes things much easier on people, so it's still done, but it is never as universally embraced as it is in our world, at least partially due to concerns over droid uprisings