My attempt #2.  OK, I got a bit carried away and it turned into a TV ad instead of a slogan.  I guess it's a bit off-topic, but if it was done right it could be a 1-minute teaser to get people interested... So, I'm posting it anyway just to add to the idea pool:

[voiceover in a somewhat spooky or sardonic voice ... i.e. not total doom and gloom, perhaps almost silently laughing at the doom and gloom
 but with brooding ambient background music, deep -- orchestral basses/cellos/bassoon etc]

The world's economy is in turmoil.  The people struggle while the fat-cats grow ever richer.
Governments, banks, and credit card companies control your money.
Heavy fees and taxes, the power to freeze or confiscate your savings at their whim;
they decide how much your money is worth and how you may spend it,
and it's only going to get worse...

[voiceover keeps with the same style but changes to a more uplifting tone
 with change in music key + higher timbres including triangle & bell-tree (ch-ching... maybe even go full-on cheese like the simpsons intro chord )]

...or is it?
There is a growing community of ordinary people, who know a better way.
Who believe that your money should be yours to do with as you please.
Now, you can send and receive money, quickly, easily, and safely, anywhere in the world, with transaction fees that are next to nothing.
It's the new global currency, which gives control back to you.

Bitcoin.  The future of money is here.  Join us.

["Bitcoin.  The future of money.", logo, and url on screen]


(...or something like that)

Obviously bitcoin is not ready for this kind of thing yet -- it needs to pass the "grandma" test for ease of (secure) use first. 
Also I'm not sure about the introduction being "doom and gloom" -- I think it would have to be done just right to grab and hold people's attention, hence the almost humorous "spooky" voice, rather than the vibe of yet-another-bloody-depressing-news-story. 

On the other hand, maybe this is the wrong approach altogether -- it has a more "anti-authority" approach than some would like, I suppose...

Thoughts?

Bitcoin is a digital currency that cannot be made worth less due to the issuing of new bitcoins by a central bank.

Different products and services enable transfer of bitcoins between bitcoin eWallets, which may have different owners

I find that a lot of people don't understand anything about money or where it really comes from. Trying to tell them about decentralization and middlemen is a waste of time! It has to be about the convenience, ease of use, and security!!![/b]

I pay my mutha uckin' rent fortnightly
Mutha uckas at the bank trying to play me
And I'm out for my account
'Cause out on A.P
On AP
Yeah, you know me
Mutha ucka charge a two buck transaction fee
Makes my payment short
My rent comes back to me
Minus a twenty-five dollar penalty
So you'll see me 'cause of your mutha uckin' fee
Read the words
On my ATM slip, it said
We're all mutha uckas
And we're uckin' with your shi-
Too many mutha uckas
Uckin' with my shi-
My transaction shi-!
There's too many mutha uckas
Uckin' with my shi-
My weekly statement shi-!
My weekly statement shi-!
Too many mutha uckas
Uckin' with my shi-
With my balance shi-!
How many mutha uckas?
Too many to count
Mutha uckas

I think what I posted in my thread works rather well and is even informative:
Bitcoin is the perfect form of digitized cash. It is anonymous, easy to use, secure, fast, and can be accepted anywhere at any time.

http://forum.bitcoin.org/index.php?topic=26322.0

+1

Governed by math,
controlled by no-one,
freedom to spend.