PlantFiles: Aren Gelora
Arenga undulatifolia

I found this website for the 1st time tonight.
I live in Cairns - North-East Australia - a very warm tropical area with rainfall averaging over 100 inches a year.
I have 3 Arenga Undulatifolias growing in my front garden and they are indeed a beautiful palm.
I planted mine about 8 years ago when they were about 1ft tall. 2 of them are now about 10 ft tall with fronds at least 15ft long.
They are a very spectacular palm - probably more suited to tropical areas and big gardens.
I would suspect that your hardiness description on the website is a bit optimistic. They are native to the Borneo/Phillipines area and would not be happy with temperatures down to 4.5 deg C.

Though I can't grow this palm in California, it is a fantastic huge palm for large tropical gardens. It has a fantastic ruffled leaflet and tremendously long leaves (10'plus) that spread quite a distance in all directions. It is a suckering palm so it takes up a lot of room. The leaflets are not only attractive in being wavy, but are large, fishtail-shaped and have an attractive whitish undersurface.

Later note: this palm performs wonderfully in Hawaii, a zone 11. The best examples of it I have ever seen are from that zone. However, there is a local grower who has had one in the ground for 5 years (zone 10b) but under a lot of canopy. Whether it will survive to adulthood remains to be seen. Perhaps this palm will survive even cooler temps than we first thought.