the goal is just to measure maximum, minimum and average value of AC mains. you can let the device plugged for a few days and see if there is much variation in the mains line or what happens when a heather/dryer/machine is turned on... things like that.

I found a doc from st about PS without transformers:

http://www.st.com/stonline/...s/8313.htm

and a nice faq with a topic about ground loops:

http://www.opto22.com/Support/faq/faq922.aspx

I even found a lot of docs about offline power supplies (the proper name for a PS withou transformer?), mostly for switched models.

but I'll stop googling the topic now since it's past 3am here and in a few hours I must wake up :-)