SiliconImage haven't updated the irSI3132 drivers for years (seriously).
But SonnetTech have and I'm pretty sure they're gonna keep that up.
I use a cheap SI3132 card with the SonnetTech drivers without any probs under Leopard, too.
Edit:
So, SonnetTech won't update their Si3132 drivers. FirmTek, however, did. Unfortunately, their driver causes kernel panics with my cheap card.
But after some research I found that there is
native support, also in SL, for the JMB362 by JMicron. (see xlr8yourmac.com)
Over here in Germany, however, there are only 1-port cards featuring the 2-port chip JMB362...
But for those of you who live in the US or are willing to order from there, I found
this.
Using a backplane adapter for the internal port, you get 2 SATA-ports that should also work under Snow Leopard.
Since I don't know for sure if it works, I'm going to order here in Germany, because it's easier to return if it doesn't. And if it does and I want two ports I could also buy another 1-port card, since the backplane adapter would also block one of the PCIe-slots.