It's important to note that it's extremely easy to "beat" comment spam if you have a relatively low-traffic site and some programming time to spend on a custom solution.
The per-message payoff for spam is horrendously low. Spammers only do it because they can post a huge number of messages. The big threats are necessarily automated, and that automation isn't going to bother with special cases for any site that isn't worth their while.
For the longest time, the anti-spam measure on my blog's comments was a field that literally said:
Type the word "elbow": _____
And it only accepted the comment if you typed the word "elbow". It wasn't even a dynamic word. It was literally hardcoded to be the word "elbow". This stopped almost all spam for years.
Somebody finally added this to their bot, so I modified it slightly, to:
Type the word "humour", but with American spelling: _____
Once again, this stopped almost all spam for years.
A few months ago, more for fun and curiosity than because I really needed it, I replaced that anti-spam field with a JavaScript hashcash-based solution. Basically, when the user wants to make a comment, the page fetches a problem from the server whose solution is difficult to compute but easy to verify. The page then computes the solution on the commenter's computer, and posts it along with the comment. I tuned it to take about 20-30 seconds on modern hardware/browsers.
For the curious, the problem I chose is a standard one you'll find if you search for "hashcash". The quick version is that the server generates some random data and gives it to the client. The client then searches for a salt that, when added to the data, produces a SHA-1 hash with a given number of leading zero bits. The number of leading zero bits required can be easily tuned, with each additional bit roughly doubling the amount of time it takes to find a solution. The client's solution can easily and quickly be verified by just combining the client's solution with the generated data and counting the number of leading zeroes in the SHA-1 hash.
Now, this would not stand up to a concerted effort. My JavaScript implementation is pretty slow, which means that the 30-second work required by my page could be reduced to I occasionally get spam, still. From looking at the logs, I'm about 99.9% sure that these spam comments are being posted by actual human beings sitting at a browser. I have no idea how it could possibly be cost effective to do this, but the quantity is low enough that it's not a real problem.
My crazy hashcash solution has an additional benefit, which some might see as a liability. I only start the work when the user clicks on the comment form, in order not to burn up their battery unnecessarily if they don't plan to leave a comment. The user then has to wait until the proof of work is completed, typically 20-30 seconds, before they can post a comment. This strongly discourages short, off-the-cuff comments, which are almost invariably worthless anyway.
In short: spam prevention is easy if your site is small and you have the time to invest in a custom solution. Any custom solution will do. As long as it doesn't match whatever patterns spambots possess, it doesn't much matter what you do, as long as it's unusual.
Once your site gets big enough, you'll no doubt need more. But cutesy stuff like changing your form variable names won't save you then anyway. If you're at the level where the linked solution works, you're at a level where nearly anything custom-made will work.