As someone said you can get a component switch that will allow to "share" the one HD input your tv has.
Really any RCA switch will work, if it is a manual (mechanically operated switch). If you want a automatic switch, you will need to look into one with enough bandwidth for HD video. I believe 30mhz is the minimum you would want for HD. I purchased one on eBay for around $70 that has 3 component and optical digital in and 1 component and optical digital out that uses a remote to change inputs. If I recall, it has 50mhz bandwidth.
Then look into an external OTA HD box. A TON of people use the old Voom boxes. Do a little eBay searching and you will see you can get one of the old Voom Motorola boxes or an older Samsung for less than $75.
About 4 months back I got a US Digital box from Walmart for $20.
So for $150 tops you could do exactly what you wanted to do and more, since the switch will still have 1 more input for your DVD player, HD video game system or whatever.
Thanks a lot guys.
First I will try the last idea and switch the component cables going to the TV from the DTV set to one of the other 2 and see if I can get 1080i. If not, then I'll check into the boxes you guys mentioned.