I am pretty sure that there is another thread either here at
OpenACS.org or on the aD web/db board that discusses/lists
these folks.
Last I recall there were somewhere between 5 and 10 of them.
My company, www.zill.net, also offers OpenACS hosting, either
using Apache+mod_aolserver OR the stock AOLserver 3.4 .
There is also Musea, Ybos.net, OpenForce, Furfly, hub.org, plus
a few others whose name(s) escape me. There is at least one
British provider as well.