The PS3 has a number of built in functions that allow it to act as a media center without any special software. Simply use a DLNA enabled program like Windows Media Player to share your media with the PS3. The one big omission is that it won't play container type file formats like AVC and MKV.
To solve this you can run PS3Media Server on a PC on your network to transcode the video and stream it to the PS3 in a format it understands. Also, MultiMan has support for AVC and other media formats and is in constant development, you may want to check it out. I too would love to see XBMC ported to the PS3, but in the mean time here are some alternatives. Good luck!
I did use PS3 media server in the past to stream wireless,but the reason I asked was that I visited a friend recently and he purchased the new Xbox, and he compared the PS3 and Xbox media capabilities and I must say the PS3 has an all text display as opposed to the XBMC format that displays your pics,music and videos in a great looking picture library including slide shows etc.
Plus the Xbox has an extender built in which helps. That's the only thing I would want from the xbox the rest is all about the PS3.