I use linode VPS. Similar to the rackspace suggestion. Although it's a VPS, it can resize in under a half hour. So if you have constant scalability requireme

I use linode VPS. Similar to the rackspace suggestion. Although it's a VPS, it can resize in under a half hour. So if you have constant scalability requirements (don't need to spin up/down during the day), I love it. I run a friends' site at MediaTemple, which handles all the scalability aspects for you. But he doesn't get much traffic (not enough to really impress me WRT their scalability), but they advertise that they'll handle as much traffic as you need.