Does any version of Android support smb natively? I use Android 2.x on my phone and will buy a Tegra 3 tablet in the (hopefully) near future.

I use files on my NAS with my phone. I can access those files with external tools (ES file explorer), but other applications cannot access these files.

I just wonder if Honeycomb or Ice Cream Sandwich have built-in support for smb?
In a word: No.

In order for all applications on the device to have access to a cifs mount, cifs support needs to be compiled into the kernel (or at least available as a loadable module).

I've never seen a device that shipped with such a kernel from the manufacturer.  Doesn't mean that they don't exist, I just haven't seen them.

Now, for some devices that have been rooted/unlocked and replacement kernels can be compiled, I've seen kernels made with cifs support built in.

Once you have cifs support in the kernel, you could use an application like Mount Manager to handle the system-wide mounts.

The applications like ES File Explorer you've seen with cifs support have their own private implementation.  They are unable to mount the share into the global namespace such that other apps could take advantage.