Demon's Souls has a been a massive hit in both Asia and the States. Last year it won various "Best RPG" awards, even bagging a few Game of the Year ones too. My only gripe with Demon's Souls is that RPG fanatics in the UK may never get their hands on it!
Heath Flor gave Demon's Souls a flawless 9.5 in his Neoseeker review. To put things into perspective, Mass Effect 2 and Uncharted 2: Among Thieves are the most recent to get the almost perfect 9.9. Demon's Souls is said to be the staple example of action-RPGs, offering plenty of replay value.
The port could easilly be made for PAL regions, but the only factor holding Atlus back would probably be the sales figures here in Europe. The continent is only host to few hardcore gamers, but it hasn't stopped other unique titles releasing. Another JRPG titled White Knight Chronicles is launching next month for all regions, which makes the situation even stranger. You expect casual gamers to invest more in action RPGs than the conventional turn-based ones, eh?
Coming back to my main point, Atlus should really consider making the port soon. With Game of the Year status supporting it, I'm sure it will be an equal success here in the UK. If it doesn't happen though, PS3 games are region free, so importing the game would work fine.