It's starting to work. I fiddled with the board connected to a windows PC. Updated the Ztex firmware (it was indeed old) and without switching it off connected it to the Raspi.
cgminer version 2.10.4 - Started: [2013-01-31 20:56:50]
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(5s):473.1M (avg):200.2Mh/s | Q:7 A:8 R:0 HW:1 E:114% U:2.7/m
ST: 2 SS: 0 DW: 12 NB: 2 LW: 27 GF: 0 RF: 0 WU: 3.0
Connected to 50.31.149.57 diff 1 with stratum as user bensguild_raspberry
Block: 04ebfa5299bc5b8d... Diff:2.97M Started: [20:57:26] Best share: 7
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[P]ool management [ S ]ettings [D]isplay options [Q]uit
ZTX 0: 04A346C253-1 | 208.0MHz | 194.9M/200.2Mh/s | A:8 R:0 HW:1 U:2.66/m
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[2013-01-31 20:56:47] ZTEX 04A346C253-1: capability missing: 0 7
[2013-01-31 20:56:47] ZTEX 04A346C253-1: Found Ztex (fpga count = 1) , mark as
0
[2013-01-31 20:56:47] Probing for an alive pool
[2013-01-31 20:56:50] Switching pool 0
http://btcguild.com:8332 to stratum+tcp:
//50.31.149.57:3333
[2013-01-31 20:56:54] ZTEX 04A346C253-1: Frequency set to 200.0 MHz
[2013-01-31 20:56:58] Accepted aa113f67 Diff 1/1 ZTX 0
[2013-01-31 20:56:58] Stratum from pool 0 requested work restart
[2013-01-31 20:56:59] API running in IP access mode on port 4028
[2013-01-31 20:57:06] Accepted 3c583a1c Diff 4/1 ZTX 0
[2013-01-31 20:57:26] Stratum from pool 0 detected new block
[2013-01-31 20:57:41] Accepted d31c8be8 Diff 1/1 ZTX 0
[2013-01-31 20:57:57] Accepted 57d37c0c Diff 2/1 ZTX 0
[2013-01-31 20:58:09] ZTEX 04A346C253-1: Frequency change from 200.0 to 204.0 MHz
[2013-01-31 20:58:18] Accepted df48cf46 Diff 1/1 ZTX 0
[2013-01-31 20:59:19] Accepted 22be4434 Diff 7/1 ZTX 0
[2013-01-31 20:59:32] ZTEX 04A346C253-1: Frequency change from 204.0 to 208.0 MHz
[2013-01-31 20:59:44]
ZTX0: invalid nonce - HW error [2013-01-31 20:59:44] Accepted a49ee248 Diff 1/1 ZTX 0
Nice
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