According to this post on Mike's blog today, it's "time to say goodbye" to curly braces.

Yeah, it may come suddenly, but it's time to say good bye to curly braces used as string indexing operator.

PHP-5.1 will issue an E_STRICT error, and PHP-6 will probably don't know this syntax any more.

I don't know why, but I've got some strange sentimental feelings, maybe because I've never used those other brackets for string indexing...

The method he's talking about here is the $string{0} format, and it's making way for a unified string/array operator. It won't break things badly, though - in PHP 5.1 it'll just toss out a non-fatal error to let you know. But watch out, by PHP6, this functionality will be gone...