15:57 Jan 21, 2010 

1. מי ישורנו/ישורם
means quite simply
מי יכול לצפות (אותו/אותם) מראש
Literally - "לצפות מראש" is "predict" in English.
The synonym "forsee" (same meaning) is particularly nice,
because it uses the stem "see" just like the Hebrew "ישורם"

2. Your suggestion "Who can estimate" lacks the future focus.
In the phrase "Who can estimate the number of casualties in Haiti"
for example - you wouldn't use מי ישורם

3. Your second suggestion - "Who could Imagine", is really used in quite the opposite way than you meant. It is retrospective!:
"Who could Imagine" = "מי היה משער"
or "מי *היה* יכול לצפות מראש"
and it is mostly used for things that DID happen, not imaginary :-)
"Who could imagine that Israel would survive the 48' war?"
"מי היה משער שדווקא בגן-עדן כמו האיטי יקרה אסון כזה?"

14:46 Jan 21, 2010 

I also think that "estimate" isn't quite right, but the problem with "forsee" is that it means "לחזות" or "לראות את הנולד" - something that has happened or may well happen - like "We forsee rain tomorrow" or "we forsaw the fall on Wall Street" and "מי ישורון" refers to something completely unthinkable that hasn't happened yet or could of happened and didn't (usually bad). That's why I sugested "Who could imagine" - it expresses the meaning in other words.

14:11 Jan 21, 2010 

Phrases or idioms using this form are NOT reserved to unlikely events/outcomes or ad ones, but to any consequences.
It means simply "who can tell (the outcomes)" - "Its difficult to guess/foretell (the consequences)", or in Hebrew - מי יכול לחזות אותם.

That's exactly what "Foresee" means - לחזות, לצפות מראש
(Please look it up :-) ==> http://milon.g.co.il/targum/Foresee)
"Foresee" is also not limited to a specific kind of outcom.

I think "estimate" really misses the part of outcomes and future.
You can estimate the present status of something.

Some examples in Hebrew:
ואחריתו מי ישורנה (bad)
but also (using plural):
צעד חשוב שפירותיו מי ישורם