I wouldn't say it's normal, but this is the first I've heard of this.
Have you tried a different Ethernet cable, perhaps one that has extra shielding? I don't know how that would factor, but may be worth checking out.
If you're not able to find satisfaction, I'd recommend contacting the Samsung customer service center for further assistance.
I bought a LE32C550 less than a week ago and so far I've been quite happy with it.
I noticed that as soon as I plug an Ethernet cable from my laptop to the rear panel of the TV, this seems to cause some interference (lines across the screen). This immediately stops as soon as I unplug the cable.
Is this the normal behaviour? It feels like something is wrong. Whenever I want to watch the normal TV (analog/digital) I always have to make sure that the Ethernet is unplugged otherwise this will result in bad video quality.