Found one contender & other options

The Zotac ZBOX HD-ID11. It's cheap, small, has Nvidia ION2, basically a 9400M with improved VDPAU decoding (no unplayable video dimension gaps).

But I'm really not enamored with the Atom. On it's own the Atom barely enough to play h.264 at 360p. So you are basically counting on that Adobe gets and keeps its VDPAU support working in Flash, Youtube will not move WebM, and to a lesser extent that MythTV will not drop h.264 VDPAU support (they've already dropped some MPEG2 decoders).

Also, the ZBOX's are noisy. Measured about 50dBA closeby, when playing 1080p.

Another option would be to build a full blown >3GHz PC plus a low profile fanless Nvidia GPU with VDPAU 'featureset C', these come in at about €70. But since a PCI-E slot is perpendicular to the motherboard, and an ATX power supply is about as big as a Mac Mini all in itself, this will need a lot more space for the computer. And then it may not even be silent.

Choices, choices..

Edit: Discussed the options, and the slightly faster ZBOX with SU2300 is more appropriate than a lot bigger & more expensive PC.