I had to poke around a bit to find instructions after getting my 2Gb card. I found instructions at http://maemo.org/maemowiki/HowTo_Support2gbmmc and followed them to install the kernel image I got at http://www.freenux.org/~mm/zImage-su-18-200627 . (However, you need to flash the new kernel from a Linux machine; evidently the current Windows and Mac versions of the flasher don’t support replacing just the kernel without replacing the whole flash-ROM image.)

I did this starting from the it2006 image; I don’t know if the 2Gb-capable kernel works with the it2005 image, but I’d be kind of surprised if it did. So if you’re not running it2006 already, I’d recommend you upgrade first. (That will mean backing up all your stuff and replacing it afterwards. However, replacing the kernel with the 2Gb-capable version doesn’t mess anything else up.)