While anyone can host a Horizon server, is this considered a semi-centralized entry point because the primary REST endpoint for most clients will be through Stellar's own horizon.stellar.com?
Are there other known public stellar hosts?
It is definitely a potential single-point-of-failure at the moment, but as we progress, more will come online and more apps will start supporting other servers.
Ideally, if you're running a service on top of the network, you would run your own Horizon server too, just as you should run your own stellar-core. Remember, that's the only way you get to pick what other validators to trust.