Technically, Bitcoin is not money or property. Bitcoin is just valuable information so my guess is that in theory it's not taxable.
If the company had to buy the Bitcoin in order to transfer it, its nature changes greatly.
Taxing authorities may not understand Bitcoin, but they do understand the idea of "widgets" that are worth something to somebody when people pay or work for them, whether tangible or not. They also understand hours of labor.
Instead of bitcoins, I'll refer to Webkins credits. (Webkins are childrens plush toys sold at retail, each one has a code that allows for online play of games at the Webkins web site... something whose value is equally as nebulous as Bitcoins.)
Suppose I had 10 software developers working for me, whom I compensated purely with Webkins virtual pet codes, and then I were audited by the IRS. Those pet codes would probably be deemed to have the same value as what they cost me, or what they might have cost somebody else to acquire on the open market. Presumably, if software developers will work for them, they are worth something beyond the mere ability to play preschool games online.
Similarly, if I had software developers working for me in exchange for dirt, the IRS would just as quickly assume that the dirt had a market value similar to the market value of software development, and assess accordingly. They rightfully assume that if the dirt were worthless, people wouldn't work for it.
Companies have to keep records on how they spent their money, so they can differentiate between "profits" (which are taxed), versus the costs of doing business (which are not). When a company gives something to its employees, that's "cost of doing business" (the tax burden is shifted to the employee), but the company must keep records to substantiate that's how it really spent the money.
The moment a company pays a large sum of money for something to compensate the employee, the company has to be able to account for where that money went. That's where it would become detectable by a taxing authority. The nature of what it was doesn't matter, it would be the same as if the employee accepted his salary in dirt or chicken feathers.
If the company made payments entirely with Bitcoins it generated itself, and kept no record of it, the "valuable information" idea would probably be more tenable, possibly because the IRS wouldn't have a place to start asking questions. Of course, possibly no different than an employee paid entirely in cash in a cash business, where the inflow of cash cannot be measured.
Perhaps it helps to think of other situations where an employee gets "paid" in valuable information.
Example: Software company A hires a developer, and pays him a fixed salary. After a few months a competitor tries to hire him by offering a bigger salary. Company A cannot afford to raise his salary, but to avoid losing him they make a deal. He is allowed to study the entire source code of their closed-source product. Previously he was just given access to the module he was working on. This will give him valuable insider information about industry practices that will potentially increase his net worth.
The question is: Will such a "raise" be taxable? If yes, how would the value of the be "raise" be calculated in USD? Would the taxman need to hire an external auditor who signs a non-disclosure agreement? What if it is a prototype that isn't on the market yet? Can the value even be estimated? How would the taxman even know about this, given that the software is secret? And so on...
"Valuable information" isn't quite the same, especially if the company didn't have to incur a directly connected expense somewhere else in order to give it up. If "valuable information" were personal advice from a lawyer, who had to be paid, then this would very well be taxable compensation. If the valuable information were the first 1000 digits of pi, possibly not so much.
The "value" of valuable information matters. Can employees use it to pay their rent or mortgage? Can it be resold? If "valuable information" is a list of serial numbers off of dollar bills and the serial numbers can't be redeemed for value, then probably not. If it's hundreds of thousands of Webkins codes that the employee can resell (and is doing so), then probably yes.
Not every perk is necessarily compensation. If Software company A decides to give the developer a faster computer or a bigger office, these are perks that would be valuable to an employee but these aren't compensation and wouldn't be taxed like a "raise". The taxing authorities understand that buying computers and office space and furniture is a necessary part of employing software developers and would routinely accept that as a tax deduction.
If the larger office space started included kitchens and beds and living quarters for the developer and his/her family, then it would start to be treated as compensation.
If the company were Webkins themselves, and were generating their own Webkins pet codes, and giving them freely to the employees to resell on the open market, probably nothing would happen at first. But if later, employees were discovered generating loads of unreported income (maybe one gets audited for having a huge house while his W-2 shows minimal income that couldn't possibly afford it, and he snitches), and the IRS was able to determine that the pet codes were really how they were getting paid, you can bet the IRS would be looking to slap a value on those codes.