Petroglyphs and Petrographs of the Philippines by Boj
Visited the Angono Petroglyphs last month (June 2011). The site is also referred to as the oldest known artwork in the Philippines.
The site is well-managed: nice landscaping, small but adequate museum facilities, a platform for viewing the petroglyphs.
However, transportation to the site is a little difficult. There's been no report on public transportation so independent travel to the cave area is not really that attractive. I ended up hitchhiking with locals. Other options would be to rent private vehicles.
Another issue would be the buffer zone of the site. A good part of the hill/mountain seems to be located within a private residential village.