I just spotted a note on Twitter, from SiliconFarmer, referring to the ATtiny44A coming in a 0.45 mm pitch QFN as well as a 0.5mm pitch MLF package. (In practice, an MLF is the same as a QFN, by the way.) Just in case you actually care, we're on twitter at "pcbassembly".
I've run across a number of 0.4mm BGA packaged parts, but this is the first sub-0.5mm QFN I've seen. Interesting that they have two different sizes of QFN package, one at 4mm x 4mm and the other at 3mm x 3mm. If you're that tight on space, that little 7 square mm of extra open area can make a difference.
Screaming Circuits won't care on the assembly floor. We do plenty of 0.4mm parts so a 0.45 isn't anything new. The most important thing to remember is to use the right footprint. It's easy enough to accidentally use a QFP footprint when you have a QFN (like here). I could see it being even easier to swap for the wrong footprint with this part. Doing so would be bad, most certainly. You might get one or two contacts per side on the right footprint, but that's pretty much as good as none.
It's like Ice-9. The same, only different.