you can use an USB UIRT to send messages to the TV. also the tv must have an interface for connecting to linuxmce.
Depending on your TV, you can have LinuxMCE control it by IR codes (using an IR transmitter receiver such as USB UIRT or GC100) or by RS232. You must add your TV as a device (if you find your model in the list when running the video Wizard, in the TV's section) or define a new device template for it (if you don't find your model there).
When you define a new device template you will be able to set the way LinuxMCE will control it and also to input your IR codes or commands.
I believe my TV has an RS232 Port (It's a Pioneer 50" Plasma (about a year old) so that sounds good, but I also have a SkyHD HiDef Satellite box so I guess the IR solution may be better anyway. I was thinking of getting the Microsoft remote for the MediaDirector box, baiscally because I thought Linux compatibility would be more likely than some obscure IR reciever I hadn't given any though to the nedd to trasmit IR as well. Maybe USB UIRT or GC100 with my Harmony 885 universal remote (which is on order) will be better.