Here are the comments of a former Utah police officer:
For my working career as a cop here (which ended at retirement in '99), the state rule was that "manner of intended use" was the measuring stick for determining what was or was not considered a deadly weapon and prohibited for concealed carry. Open carry was anything goes, up to and including swords (used to encounter a guy who carried one openly slung over his back, we talked to him a couple times, he was calm & reasonable & not bugging anybody so we didn't bug him).
No specific blade length limits for concealed carry in either folders or fixed blades.
Technically, you could be arrested for carrying a Bowie knife up your sleeve (and we did that a couple times), but for a conviction the prosecutor would have to convince a judge or jury that the preponderance of the facts and the situation in general indicated the knife was being carried as a deadly weapon and not as a general purpose utility tool.
Your chances of being arrested and convicted for a violation under state statute would increase with the size of the knife (big), the type (Rambo-style), and how you were carrying it (up sleeve, in boot, etc.). The circumstances where you were encountered by police would also be taken into consideration (in a gang setting, during a confrontation with police or others, etc.) as well as any statements you may have made in the presence of others.
Something like a four-inch hunting or utility blade in a regular sheath on your belt under a shirt or jacket that was accidentally seen and called in to police while you were shopping, for example, would be much easier to explain as part of your everyday tools at work where you cut strings, ropes, or packing materials, than a 12-inch Bowie in your boot and a beer in your hand in a dark parking lot making threats such as "I got something for you here you don't wanna be messin' with!", or "I got something I carry just for *******s like you!" to one or more people in an obvious hostile and aggressive manner. The same criteria for determining an item to be a deadly weapon and carried as such in violation of state law could (and did) apply to screwdrivers.
Switchblades were not prohibited as such by state law.
The state may have changed its definitions since '99, and local municipalities have their own laws, which can be stricter than state statutes, but not less strict.
Hope that helps.
Denis
http://www.knifeforums.com/forums/showtopic.php?tid/379799/
Also be aware that the State of Utah has now done away with all local knife laws with statewide preemption:
http://www.kniferights.org/index.php?option=com_content&task=view&id=141&Itemid=29
It appears to me that you can open carry just about anything without a permit, and conceal carry anything with a permit. In between, you can carry concealed without a permit if it is not carried "as a weapon". As shown above, switchblades are not prohibited.