Johannes Schluter briefly touches on unicode in PHP6 in a new post to his blog and how being able to use it in identifiers can come in quite handy at times.

Consider you have an application tied to an environment with a special terminology, then translating this terms to English might be extremely confusing (especially as programmers often don't really know the correct terminology of that domain) and it's good to call the thing by it's name [...] The purpose of this were some bad news: That's nothing new. The relevant scanner rule hasn't changed since 4.0 - the only change is that PHP 6 doesn't treat it as random set of bytes anymore but knows about Unicode codepoints and interprets is as such.

After digging around a bit in some of the commit history of PHP, Johannes also found that the scanner rule (what parses the identifiers) hasn't changed since around 2000 and, apparently, won't be changing for PHP6 either.