Raspberry Pi anyone?

I don't see why not. I haven't tried XBMC on mine yet, but have run X and Chrome for some Kiosk displays just fine.

I've streamed from an Ultra 6 to a Raspberry Pi running XBMC and seen satisfactory results. The Ultra 6 is connected to a Netgear GS724Tv3 switch with a Netgear WNDR4500 router. The Pi has an Asus USB-N10 WiFi adapter. I would suspect the content the Pi can accept would fit within the bandwidth limitations of the Sparc series.

This sounds positive!
What about online tv? Iplayer, 4od and such?

I just posted about an issue I'm having with my Pi setup streaming from my ReadyNAS NV. It's an odd issue and I'm not sure what it will take to solve it.

However aside from that I can report very positive experience with the overall Pi/ReadyNAS setup. Including the $35 cost of the Pi I have about $80-$90 total wrapped up in my build. You need to be sure you have a reliable power supply that gives you a steady 5v and 1.0Amps or better. Some of the cheap Chinese knock-offs only give you about 4.5v and that will cause problems with the Pi. You need at least 4.85v that doesn't fluctuate too much so spend the extra $$ on a name brand from a reliable supplier.

You can save money by using any old SD card you have lying around if you include a reliable USB memory stick in your install. This is faster and less prone to crashes anyway. You still need an SD card but anything will do. If you go with the SD only install then you will want to get a good class 10 SD card at least 8GB. Check their compatibility list. I also added a EdiMax USB WiFi adapter which worked out of the box. And finally I bought a really nice case from Cyntech for around $10 incl shipping. This is not necessary but with it I was able to mount the Pi on the wall out of sight behind my television. And it's a cleaner looking build.

My ReadyNAS is anything but fast when it comes to copying files to or from it... but streaming files to the Pi using XBMC works beautifully for me even over WiFi. I just have to solve that strange reboot issue viewing the associated movie/episode information in the XBMC menu system. That seems to be related to my ReadyNAS and so far I know of no one else who has experienced this issue.

HTH

I have a ReadyNas Duo and streaming with UPnP to the raspberry works fine. The raspberry will however not play any video format. But the raspberry can also mount filesystems (samba or NFS). If you like to use tags (in your mp3's) UPnP is the way to go but if you have a sensible directory structure or if you have no tags (as with video's) mounting is reliable and fast. I personally don't like Minidlna so much because it acts weird if I add a new directory to the scanned tree. (the database get mixed up)

I have a Readynas Pro 6.
I use the Pi with 2 stage boot, using SD card and USB flash drive as mentioned above.
I have no problems at all streaming all content.
I have upgraded the processor on the Readynas, (I have version 1)
That made all the difference. Before I could stream most 720p movies without buffering, but some would buffer, and all 1080 would buffer.
After the ram and processor upgrade, no problems, except on some MKV files, that issue has to do with the way the file is encoded.
IT will cause the PI to reboot.
Just convert the file to MP4 and there will be no more issues.