anyone seen weird number format issues? I have some users trying to type in '100' into an amount field, and it's spitting back '10,00' but is ok if they enter '100.00'

check their user preference setting

yep tried that

client script interfering?

good thought @Shawn Talbert - though it's happening across many interfaces/pages, including the Automated Cash Application page, which i dont believe can be scripted against