I think it's OK to post this because it's already clear that null bytes can be entered into posts and comments, and possibly other fields.
If one takes a look at this answer , it's clear that one can use null bytes to circumvent minimum character limits. However, this can create problems well beyond circumvention of minimum character limits.
This is an instance of CWE-158 and is a potential security risk, creating the risk of attacks such as poison null bytes (CWE-626; see also Hakipedia article). The Stack Exchange team should strongly consider stripping any null bytes from any field where a user can enter text at the time it is submitted.
Edit: For those who aren't seeing the null bytes, I suspect that the browser is neutralizing the null bytes, replacing them with spaces. Firefox 15 on Windows 7 is rendering the null bytes. You may be able to see them if you use a program such as Wireshark to read the raw HTML as you load the page.
Edit 2: I'm not able to see any null bytes in Wireshark. I got null bytes when I pasted it to HxD, but when I pasted it into Notepad++ with the correct encoding, saved the file, and opened it in HxD, I saw proper Unicode
e2 80 8b. It turns out HxD did not read the Unicode correctly, so this is status-norepro.