Actually there is a good amount of spacing between the HV and 5V. I have a few products that have the traces of voltage much closer! Everyone I work with in electronics is very careful and I am used to up to 480V power, so HV doesn't bother me at all, I just know to be careful.
I understand what you mean about doing a seperate box, however then there is no point of a shield at all, the only actual logic on the board is the arduino pins to drive the opto-couplers. Everything else is HV, so those traces are very small and only 5 of them and a few grounds. Maybe I will play around with the idea of a seperate board, I am already building one for a different project.
I personally like the idea of having everything on one circuit. I can put this shield on a arduino and put it in an enclosure and I have 4 circuit control in a very compact package with dimming too.
I like the relay squid, but for simple things it is just too darn expensive for me!