Hate to reply to an old drawn out thread, but it seems the right place and I can't find the answer anywhere else...
I have no trouble creating and loading the custom POI's, multiple databases, custom icons, etc. (thanks to all who posted above!) However, the multiple line comment seems problematic. I'm creating .csv files and using the POI loader to create the .gpi file. According to the POI loader helpfile, if you enclose the POI name or comment in quotes you can include page breaks, and this is correct; it works. However, it seems you can only create four lines either this way or if you enter a long line and let it wrap; anything after the fourth line is ignored, even though the GPS (at least my 60CSx) appears to have room for eight lines of 18 characters each (numbers or uppercase; lowercase gets a few more characters per line).
What's interesting is that if I open up the .gpi file in a hex editor the additional information is there, so presumably it's the GPSr that's not displaying it.