Is it possible to use an AmigaOne computer or several AmigaOne clusters as web server?
Yes this is easily done using Linux. Just compile Apache and away you go. Use a Cisco CSS1100 for load balancing and/or session persistance.
On OS4 you will have to check out the Amiga Apache folk who I believe anounced that they would be supporting OS4 - check out their site. Again just use a CSS1100 for your LB/SP requirements.
In these dot.com bust days you can pick up CSS1100's on ebay quite cheap.