It’s available, as discussed in other threads:
It offers a lot of new possibilities, see here:
I suggest to check on a local development web server (LAMP, XAMPP, and so on) if your applications are all compatible and then switch because not only PHP 5.3 offers many new useful features, but it’also faster. I’ve seen speed increases about 50%! Obviously it depends from what your application does, but the thing is really faster… which also means less cpu seconds usage on your account that’s is so important.