Jarn, when I look at the math, it seems marginal at best. (for the record, I'm one of those crazy fatwallet guys juggling about 5 cards right now, and planning on applying for 10-20 soon)

Here's the way I see it.

1) Take ING's 4.6% return.
2) Lop off 33% for taxes, get about 3%
3) Subtract the 1.9% interest rate on the card
4) Subtract the BT fee
5) Subtract the marginal reduction due to you probably not getting an entire 12 months, and having to make minimums.

... it may not be worth bothering with this if you're only talking about a 1% spread.