Welcome to Geekhack! We're always gentle (at least I am up to post count 10).
One keyboard that has a rubber dome yet claims 26-Key rollover is the new Microsoft Sidewinder X4.
Unfortunately they screwed up the design. It has a problem called ghosting where keystrokes register that you actually didn't want.
This is WORSE than key blocking so I don't recommend you buy it.
From the ExtremeTech Review.
I am the researcher behind the Sidewinder X4 anti-ghosting technology. I wanted to clear up some misconceptions I've noticed in this forum.
The "ghosting" referred to in the quote above is not from the keyboard. That's a software bug on the PC side. You can see the same behavior with competitive USB keyboards that report large number of keys. For what it's worth, I have only seen this behavior when you exceed 11 simultaneous keys from a single keyboard, so it's probably not much of an issue for most people. This is all discussed on our web site:http://www.microsoft.com/appliedsciences/SideWinderX4.mspx
On that page, you'll see that we fully document what X4 can and cannot do. The people in this forum seem to be unusually well informed about keyboard issues, so I hope you'll appreciate having all this information plainly stated.
In addition to this, there has been some misinformation about how X4 works. We add a fixed resistor in series with each key. Every resistor is nominally the same value, although this is not critical. Ultimately, the system is just making on/off measurements, checking to see if a current exceeds a simple threshold. I know this is quite an innovation in the keyboard space, but this type of technique is how many multitouch systems work and has been used for decades in industrial sensors (e.g. Tekscan).
In order to help cut through the confusion, we've put up a browser based keyboard tester. You can find it here:http://www.microsoft.com/appliedsciences/KeyboardGhostingDemo.mspx
It's capabilities will depend somewhat on what browser and what operating system you are using. But it should be good enough to give most people a quick way to test their keyboards for obvious problems. Here's what I do when I want to show just how bad most keyboards are:
Press and hold ASDW. Now try pressing every other key while continuing to hold ASDW. You will probably find a number of keys that don't work. When you find these, try releasing some of the ASDW keys. You will probably be able to narrow it down to specific 3-key combinations that don't work.
Anyhow, I hope this is helpful!