I am still waiting for my raspberry pi as well as for my first fpga mining hardware.
I agree with you about the high investmemt costs, first I waited but then I ate the sour apple and ordered some fpgas from enterpoint.
I am thinking of using a fanless Igel thin client with 1GHz cpu as fpga controller, but it draws about 20W. I have two spare ones to sell btw.
Perhaps the Bifferboard http://www.bifferos.co.uk/
can be used to run the mining software on openwrt. The board has 150MHz x86 CU, USB, NIC, 2mb ram, 8mb flash, and draws just 1W. priced at £35.
Another cheap alternative could be a TP-LINK TL-WR703N 150MBit mini WLAN router or other openwrt-supported hardware.
also see https://bitcointalk.org/index.php?topic=76685.0