The manual points out that the PN42C450 only plays music files and pictures (Pg. 4).

There are a few differences in the 4-series between plasma and LCDs, and why the decision was made is something I can't address. I have no idea why features in one lineup of technology might vary with the lineup in another.

The 4-Series is the entry level for our plasma lineup.
The 3-Series is the entry level for our LCD lineup.

Since more models exist for the LCD offering this year, the differences also vary, and that's likely what happened here. ConnectShare with Video doesn't appear on the 3-Series LCDs, so it looks like a feature that isn't available on either Entry-Level Series in 2010.

That's the best I've got.