They won't terminate your contract, though they technically could. They likely wouldn't care much if it wasn't excessive, and if it was, they'd send you a letter first telling you they tacked on the $20 tethering fee, informing you that its not free and you have to stop doing it if you don't want to pay extra. You see, they advertise tethering, hotspot, etc, but don't brag about the fee alongside it. So to many people it probably isn't clear that tethering requires an extra fee, its not like there was a huge ad campaign telling people they had to pay for it.
Pdanet (usb tether) claims to run undetected, but only works on Windows AFAIK. Maybe Macs, but certainly not Linux desktops or wifi mobile devices. Barnacle and Wifi Tether both simply act as a data middleman, I don't see how they can detect it easily running in ad-hoc mode. After all, there are other scenarios where wifi and data are both in use, such as when you're online on your mobile data connection, while printing a document via wifi.
That being said, they run the network, and can most likely tell when you're using any kind of connection sharing, "hidden" or not. I would expect that these programs don't automatically send up a red flag, and they probably don't bother to figure out what you're doing unless they have a reason to do. So if you're not using a ton of data, or frequently using infrastructure mode, I wouldn't expect they'd bother to look deeper into your account. But I would expect that they can find anything and everything you do if you give them reason to. Its just like when people try to hide things from their home ISP; they can see everything no matter what DNS, ipblocker, firewall, proxy etc., but aren't likely to bother looking if you're relatively discreet, and what you're doing isn't actually harming them or their network. Sure Verizon would love to take every opportunity to charge you an extra fee, but they're not ATT (yet) so I don't think you have anything to worry about if you tether once and awhile in a pinch.
But don't take my word for it.