Many TV's these days offer discrete commands for the different HDMI inputs (and two or more HDMI inputs are pretty standard). A universal remote would probably be the ideal solution in that case. A sensing switch would be an alternative, but if both components are turned on most of the time you still end up needing to use a remote to manually change inputs on the switch.

You have a Model 990, though, so I may be overthinking this a bit. You can use the 990's DVI switching for the PS3 and DirecTV receiver. You may have to switch the TV over to component for any other video sources, but how you accomplish that (universal remote or a fairly simple command for the TV perhaps) may not be too copmlex. The RS232 port probably wouldn't be able to help you, though.