Most probably there's no reiserfs driver module compiled for the kernel. I haven't heard that is's supposed to be better than ext2 (which, btw, probably is better for the card than ext3 because ext3 mostly adds a journal). So, your options are to either compile a reiserfs driver for the kernel (I don't know if it would compile/work ok or not -- I haven't tried), or just go with ext2 instead.
Yeah, ext2 is better for the card, but ext3 is better for your data; journalling means less losing your system and reflashing. Reiser would also be better for your data (and harder on the card), but saves space by storing small files efficiently, making it a much better choice for an OS partition. But I haven't seen a reiser driver either -- I'd expect it to compile with no trouble, but you never know till you try.
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