According to this in image.bbclass, the rootfs size should be IMAGE_ROOTFS_SIZE unless the actual size * (IMAGE_OVERHEAD_FACTOR = 1.2) is greater than that.  The IMAGE_ROOTFS_SIZE used in the comparison doesn't take the value of IMAGE_ROOTFS_SIZE_ext3 when used, as it seems it should:

ROOTFS_SIZE=`du -ks ${IMAGE_ROOTFS}|awk '{size = $1 * ${IMAGE_OVERHEAD_FACTOR}; print (size > ${IMAGE_ROOTFS_SIZE} ? size : ${IMAGE_ROOTFS_SIZE}) }'`

So IMAGE_ROOTFS_SIZE_ext3 seems to be getting ignored -
the only way I was able to get a 1G image was to use the straight
IMAGE_ROOTFS_SIZE e.g. using

IMAGE_ROOTFS_SIZE ?= "1000000"

instead of

IMAGE_ROOTFS_SIZE_ext3 = "1000000"

in emenlow.conf.

Here are the image sizes with IMAGE_ROOTFS_SIZE_ext3:

-rw-r--r--   1 trz trz  582105088 2011-03-22 01:58 poky-image-sato-live-emenlow-20110322065226.hddimg
-rw-r--r--   1 trz trz  581939200 2011-03-22 01:58 poky-image-sato-live-emenlow-20110322065226.iso

Here are the image sizes with plain IMAGE_ROOTFS_SIZE:

-rw-r--r--   1 trz trz 1032947712 2011-03-22 02:21 poky-image-sato-live-emenlow-20110322071321.hddimg
-rw-r--r--   1 trz trz 1032779776 2011-03-22 02:21 poky-image-sato-live-emenlow-20110322071321.iso

The same thing is probably true of the other variants too e.g. IMAGE_ROOTFS_SIZE_ext2 but I haven't tried those.