Ok, so nobody really knows,
I can see that nobody really knows the answer. That's OK, because I have looked on several other boards and nobody knows there either.
Here are some facts:
Some W203's do have problems with their key transmitters, there was a bad batch and there is a specific service bulletin out re that. Some folks need the newer rev keys. I don't know the exact symptom of that prob. But I don't believe it is "just" RF that doesn't work.
When properly functioning the W203 keys will work from 30 feet easily, without even pointing at the car. No, you never need to use your "head as an antenna". and RF Interference is not an issue.
If the key is working "IR-only", then you have to point it like a TV remote control, directly at the doorhandle on The W203. A few feet max dist.
The RF problem I have is apparently quite rare, "Bruno" the fellow that is on this board "on an island somewhere, three days from a MB shop" is one of the few (certainly less than 5) other cases posted on Benz forums worldwide. No, my dealer's Star Diag System hasn't found it. But hopefully tomorrow they will find another part worth replacing. I am loosing faith. My MBZ Service Advisor actually mentioned "Sun Spots" once, Good God. But maybe it was the MBZ chassis assembler staring at the sun too long.