I think, by putting all of our collective heads together here, we're getting closer and closer to a brilliant idea.
Flattr wants Euros in, and Euros out.
PayPal provides that service for them just fine right now.
Moneybookers does too.
From what the admin of the Flattr forum seems to be saying is:
If there were a "payment processor" who accepted Bitcoin, yet provided Euros to us,... we could pretty easily ADD ONE MORE PAYMENT PROCESSOR.
I just today contacted the founder of MtGox. I told him:
MtGox could be "the PayPal of Bitcoin"....
If MtGox could add a Bitcoin --> Euro trading capabilities, AND
Flattr could be convinced to add MtGox as another "payment processor"...

It's perhaps just me but I thought that everybody knew that. It seems trivial that what Bitcoin need is a way to convert bitcoin to euros and back. I realize now that, indeed, I've never saw a discussion about that on this forum.
As far as I understand, MtGox is currently only making the link between bitcoin buyer and bitcoin seller. In order to become the BitPal, MtGox would have to :
- establish a fixed rate for everybody
- establish a stock of both BTC and . (it means taking a lot more risks)
If MtGox would allow to pay directly for BTC and retrieve them, without any hassle and with a simple VISA card, that would be awesome.
A lot more website would also accept BTC if they knew that they could easily convert BTC for good old euro on their VISA account in a few clicks.