What about DTV? The transition to DTV has been delayed, but in many areas broadcasters are jumping the gun. Your TV shop should be able to tell you more on this without paying any money.
Another test would be to try using a VCR or DVD player against it. If those work then it may be an issue for the "tuning section." (we once had a TV that failed in the tuning section, but would work well against the VCR. So we used the VCR tuner section and just left it lined up to the TV. We used that TV that way for almost 5 years afterwards.)
The fact that you don't get ANYTHING does not bode well, however. Maybe a failed power supply. Today most power supplies are of the "voltage doubler" type (otherwise known as "solid state") and have no transformer. But the downside is once they go, they're done for - and they're more touchy than the old transformer types.
TVs are of the sort if they work for the first three minutes, then they'll probably work for three (or 30) years. Your case proves the exception exists.
$30 diagnostic, especially if the diagnostic is applied to the cost of repair (a rather nebulous subject I know) is probably worth it for an 18 month old TV. And failing in a repair, your replacement TV will likely be either a plasma or LCD and will be more money than $280.
Joe