I'm not sure I'd be very interested in HDMI connectivity on a phone. I'm much more interested in the direction Apple seems to be taking with the AppleTV and AirPlay.
With HDMI on a phone, not only you have to go to your TV to plug your phone to it, but you then can't use your phone anymore or switch tasks easily. Also HDMI cables and connectors are usually pretty stiff which doesn't really work well with a light device. What I mean by that is that your device is kind of flying around on your TV stand until the cable finds a good position. (this doesn't happen with heavier things like a BluRay player)
AirPlay, in principle, (I haven't tried it yet) would bring the same benefits wirelessly.
You dock it into a monitor and keyboard, and your data and environment are all there. People used to do this with laptops.
It is also cheaper (or, alternatively, you'd spend twice as much), and you can only use one at a time anyway (you can still make calls, using a mic/speaker or headset).
Of course, cloud-computing also promises same data/environment... but I think network reliability is not quite there yet, and public trust in it is a step beyond that.
Sure, phones might replace PCs at some point. However, phones with HDMI outputs are already out there and the current use for HDMI is in my opinion impractical.
What you're describing is putting your phone on the desk, next to you while you can interact with it directly with a keyboard/mouse. But I haven't seen a phone that's designed to be completely controlled this way.
The current use of HDMI is pretty much only to watch pictures and videos on a TV, isn't it? In which case you're on one side of the room on the couch, away from your phone. The phone itself still being the only thing you can use to interact with it. There might be a few models out there where you can already do what you describe, (I'd be interested if you know of any) but for the most part, this is not the case yet.
Current example: Flip cameras have an HDMI output. Which makes sense I guess. However it's actually very inconvenient to use: going back and forth between the TV and the couch to select a video...
I note that you're acknowledge the future possibility, and discuss the present. I just wanted to record these thoughts:
To recap disruption: A new technology appears that isn't as good for the users of the incumbent technology, but it has different qualities which appear to other users. Over time, both approaches improve. But a curious thing happens: the incumbent technology overshoots the needs of its users, and the new technology becomes good enough... yet also has those other qualities.
I think it's inevitable for PCs to get smaller, and the phone form-factor seems to have traction (making parts available, establishing standards etc). Of course, it might not be an actual phone, but just borrow the same technologies. It's also possible for wireless docking (to monitor and keyboard). Of course, you might have two monitors (one for relaxing, one for work), as most people do these days.
Skate to where the puck is going to be, not where it is.
With HDMI on a phone, not only you have to go to your TV to plug your phone to it, but you then can't use your phone anymore or switch tasks easily. Also HDMI cables and connectors are usually pretty stiff which doesn't really work well with a light device. What I mean by that is that your device is kind of flying around on your TV stand until the cable finds a good position. (this doesn't happen with heavier things like a BluRay player)
AirPlay, in principle, (I haven't tried it yet) would bring the same benefits wirelessly.