If HTC will have its own OS, it will definitely compete with Google's Android, and this is something bad considering that HTC is making devices for Google (Nexus One). The relation between these two companies can get cold really fast if this is going to happen, considering that HTC is the main factor for Android's impressionable ascension on market.
I didn't even mention Windows Mobile because it is clearly going to...nowhere. Microsoft announced Windows Mobile 7 recently, which won't be having an unimportant feature (in Microsoft's vision), but one which everybody uses, "Copy & Paste". Additionally, Windows Mobile 7 won't have multi-tasking. And will be quite incompatible with earlier versions of Windows Mobile. So, in my opinion, Microsoft is going to destroy Windows Mobile....
HTC said that it thinks about its own OS because it want to reduce its dependence of outside developers. Wait a second...what dependence ?! What is HTC talking about ? Dependence on Android ? I don't think so. On Andoid, HTC can do what it wants. It implemented its Sense UI, and can implement many others, and even contribute to market.
I think that HTC should think again about this, because, Andoid is the best option for their devices (I don't think that they will be able to create something better, for the customers).