One of my favorite things about the Urban Arrow is the spring-loaded infant car seat adapter (buy it here) that bolts to the inside of the bike frame. This is designed around Maxi Cosi infant car seats with specific stroller attachments. The adaptor is $250 in the US and really makes the Urban Arrow the ultimate family bike allowing you to ride with kids at a young age.
Below is a crowdsourced list of seats that have worked. I will update it as I find new information.
Mico 30 - works very well
Nuna Pipa - Very light, easy to carry in and out of the house and upstairs. It doesn't have handle release, so it requires two hands to get out. Very nice infant carrier. Extra car bases are very expensive.
Pebble - works very well
Cybex Cloud Z - Works very well