Interesting thread.

Thing is, often when the call for milk goes out the people that are willing to do it are most likely part of a natural parenting community, so even if you don't know them, someone you know does.. You know they have the same ideas about BF as you do, which is why they are willing to donate milk. It's not like they are just completely random strangers..
It depends on the situation. If it was a gift directly from a woman to my baby I would be guided by my knowledge of her lifestyle and her knowledge of her health. If it was from a milk bank, then I would want it screened because I wouldn't have access to information about the donors in the same way that I would in a more informal donation.
These 2.

Not having been in a situation to comment on, it's hard to say - I might think differently in the future. Right now though, I would happily use (and offer) breast milk from an unscreened close friend/family member who I trust and know their lifestyle. However if the milk is from a stranger I would absoloutley be using a milk bank.

My younger sister was born prematurely and my mother's milk hadn't come in, so the hossy had to use donated breast milk - parents were told formula was just uncomparable. At one stage there was no breast milk to use, so they used formula and my sister's health deteriorated quickly and they thought she wouldn't make it. As soon as breast milk was sourced, she perked right up. In that situation I think I'd be inclined to use unscreened milk. Once she was out of hospital (4 months later) she was FF and is now a strong, healthy 17 yo.