Right now, I use myopenid.com, which works just fine, but with Google, Facebook, etc. now providing OpenID, I'd like to merge all my IDs into one. Is this possible or am I missing the point of OpenID completely?
Well, in theory, you'd be able to use any openID to login to any openID enabled site, so you'd never need to have more than one... Unfortunately, the major players (facebook, google, yahoo) are only Providing OpenID, Which means you can use theirs, but it's not full OpenID support.
The real point of OpenID is to have a single login that works everywhere, unfortunately, we're not quite there yet...
Generally you'll want to pick one that you feel represents your identity. Unless you want your identity to be owned by a company, the best option is to setup OpenID delegation so that you can potentially switch providers but use a domain name you control to represent yourself on the web.
For example, I'm currently using Verisign's PIP as my provider, but I use my personal domain aaronhockley.com as the URL that represents my identity to OpenID relying parties.