This has only occured with images that I uploaded today. Other pre-existing portrait images have are not affected.

Some portrait images are being "squashed" down (and/or stretched out) in the "image" view, but appear fine when clicking the image to enlarge.

I don't know why this is happening.

Link: http://bit.ly/wu5dZY

FYI I upgrade to v1.4.2.2 before uploading these images.

Did you try clearing caches (ZP and browser)? The only thing I could imagine that those images do not store their size.

I cleared ZP cache and browser cache. Images shot using Nikon D90

Correct: http://bit.ly/w4TEFA
Incorrect: http://bit.ly/wu5dZY

They both look correct to me.
The browser cache can be very persistent.(I learned from experience)
Try clearing it a few times more.

I did a "metadata refresh" -- could that have solved it? they both look ok to me too.

Quite possibly the metadata refresh could have soved it. That would "reset" the image dimensions. So if somehow they were wrong it would fix it.