I am not sure but I have a feeling that the WizNET chip does not "play nice" like other SPI chips do. They are supposed to hold the MISO line floating when not selected but I seem to remember that this chip does not and always drives it either high or low, thus conflicting with any other SPI devices you may have hooked up.
But I could be wrong.
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The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected