Nokia-N900:/# sfdisk -l /dev/mmcblk1

Disk /dev/mmcblk1: 121008 cylinders, 4 heads, 16 sectors/track
Warning: The partition table looks like it was made
  for C/H/S=*/255/63 (instead of 121008/4/16).
For this listing I'll assume that geometry.
Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0

   Device Boot Start     End   #cyls    #blocks   Id  System
/dev/mmcblk1p1          0+    481     482-   3871633+  83  Linux
/dev/mmcblk1p2          0       -       0          0    0  Empty
/dev/mmcblk1p3          0       -       0          0    0  Empty
/dev/mmcblk1p4          0       -       0          0    0  Empty
Nokia-N900:/# mke2fs -t ext3 -m 0 /dev/mmcblk1p1
mke2fs 1.41.3.maemo0 (12-Oct-2008)
Could not stat /dev/mmcblk1p1 --- No such file or directory

The device apparently does not exist; did you specify it correctly?
Nokia-N900:/#