A little more digging and with the help of the link you provided I managed to get the internal device to mount. It is in the wiki but it's buried in another topic. http://www.plugcomputer.org/plugwiki/index.php/Enabling_UBIFS#Why_UBIFS_instead_of_JFFS2.3F
I'll walk through it in a bit more detail just for fun.
First, make sure you have a kernel with UBI support. I imagine if you used the Installer you're running one of cbxbiker61's excellent kernels which has the support built in.
Next, you should be able to see the mtd devices in the plug by executing:
root@sheeva:~# cat /proc/mtd
dev: size erasesize name
mtd0: 00400000 00020000 "uImage"
mtd1: 1fb00000 00020000 "rootfs"
We see that my "rootfs" on the nand is on mtd1. Now we need to attach the mtd device. This can be done in a number of ways. One way is to do it in the kernel parameters at boot time but I find the ubiattach utility the easiest. See more info here: http://www.linux-mtd.infradead.org/faq/ubi.html#L_attachmtd
To attach mtd1 to ubi device 0, use:
root@sheeva:~# ubiattach /dev/ubi_ctrl -m 1 -d 0
UBI device number 0, total 4055 LEBs (523192320 bytes, 499.0 MiB), available 0 LEBs (0 bytes), LEB size 129024 bytes (126.0 KiB)
The dmesg output after running the above command should look something like this:
UBI: attaching mtd1 to ubi0
UBI: physical eraseblock size: 131072 bytes (128 KiB)
UBI: logical eraseblock size: 129024 bytes
UBI: smallest flash I/O unit: 2048
UBI: sub-page size: 512
UBI: VID header offset: 512 (aligned 512)
UBI: data offset: 2048
UBI: attached mtd1 to ubi0
UBI: MTD device name: "rootfs"
UBI: MTD device size: 507 MiB
UBI: number of good PEBs: 4055
UBI: number of bad PEBs: 1
UBI: max. allowed volumes: 128
UBI: wear-leveling threshold: 4096
UBI: number of internal volumes: 1
UBI: number of user volumes: 1
UBI: available PEBs: 0
UBI: total number of reserved PEBs: 4055
UBI: number of PEBs reserved for bad PEB handling: 40
UBI: max/mean erase counter: 3/0
UBI: background thread "ubi_bgt0d" started, PID 22880
Now that the device is attached (you should see /dev/ubi0 and /dev/ubi0_0 entries if you ran the above code), we can mount it with:
root@sheeva:~# mount -t ubifs ubi0_0 /mnt/nand
root@sheeva:~# mount -t ubifs ubi0:rootfs /mnt/nand
I prefer the second one as it is easier to read.
Finally, you can unmount with:
root@sheeva:~# umount ubi0:rootfs
And detach ubi device 0 with:
root@sheeva:~# ubidetach /dev/ubi_ctrl -d 0