A lot of hosts are still weary of PHP5, atleast until more stable releases are made. Hosts tend to upgrade PHP in packs, as in they all do it near the same time.
PEAR and SMARTY would need to be offered via the host. Most hosts support PEAR, although I am not sure about SMARTY. Generally speaking, when you are looking for a host that offers a lot of specific extensions, you'll be looking at yourself (dedicated server). Although I could be proven wrong on that last sentence, I'd like to see evidence.
If you ask the host, a lot of time they will divuldge what extensions they allow.