Well there’s the question. I’m sure you’re asking “why?” I have a laptop in a barn and the Linksys WAP 54g won’t reach from my window to the barn, which is about 120 feet. I can use either a Belkin 54g card, or an Orinoco 11b card (Ori works better somehow) and neither one of them gives me the speed and reliability i need. So that is why i want to run a straight line of plenum a few inches underground from the Linksys Gigabit Switch out to the barn. Is it safe, other recommendations, wanna do it for me?
You might be able to get away with direct burial of standard Cat 5 cable, but at the very least, you ought to enclose it in some sort of water-resistant conduit otherwise it may not last more than a year or so. It’s also worth mentioning that there is now available specially-insulated Cat 5 cable specifically intended for direct burial without the need for conduit, but it’s rather more expensive. Some information here.
The closest reference to your situation in the 2005 NEC would be article 830 Network-Powered Broadband Communications Systems. Beyond that, 110.3 (B) requires that all equipment shall be installed and used in accordance with any instructions included in the listing and labeling. If your cable is listed and labeled for direct burial, then go right ahead.
830.47 has a chart which indicates the appropriate burial depth for direct buried cables and those in metallic conduit and nonmetallic raceways. One- and two- family dwelling occupancies show a minimum of 12" burial depth for direct buried cable, or cable in a nonmetallic raceway.
I’ve had Cat 5 plenum in the ground at home for telephone for three or four years.
I use a cable labeled “direct burial” at work for outside runs. It has grease inside the sheath. Nasty to work with. 
If you are going through the trouble of digging the trench, PVC conduit isn’t that expensive, so you might as well take the extra precaution. It will look more professional and boost your ego as well!
If the cable has a PVC sheath around it to begin with, why bother with the conduit?
Unless it’s specifically designed for direct burial, the PVC insulation typically used for low-voltage applications is surprisingly porous, due to pinholes and microfractures that are the result of the manufacturing process. This lets water in, which is bad for communications cabling. That’s why telephone cables are pressurized with dry air. Using a rigid PVC conduit will keep the water out.
Despite mine being in the ground three years, I’s stick in in PVC if the run isn’t prohibitively long.
It sure would make it easy to run a new line in the future.
If I was already planning on digging the run, I’d drop in a cheap conduit and a pull string line for future needs.
120 feet isn’t that long. You can boost a wireless signal a whole lot cheaper and easier than that. Special antennas made for that purpose cost about $50 or less. There are other ways that you can boost a signal basically as far as you want.
Ditto.
Psh. Men dig trenches.
You can get Cat5 for about $7/100 feet. That’s cheap enough I’d probably just bury it without a conduit. But I’m a cheap bastard.
I’d try some better wireless equipment first. You may experience ground loop and lightning problems running copper between buildings, fiber optic cable is the safer way to go.
yoyodyne’s links look nifty, but try slapping one of these on your router first. (from this site).
I’d try paper + tinfoil before breaking out the shovel (or your wallet).
They also sell directional wireless antennia, some IIRC can go miles.