I noticed a few people on #msx@rizon had trouble getting openMSX working with the IDE extension, so I'll just take the liberty of explaining the surprisingly simple procedure. Especially handy for those who want to view the new Dragon's Lair movie, which doesn't quite fit on your average disk image.
Ingredients:
1x suitable computter

1x openMSX v0.6.0
1x MSX-DOS2 disk image
1x Sunrise IDE Bios ROM (IDE240.DAT)
[here]
1x Sunrise FDISK 3.1 (IDEFDISK.COM)
[here]
First put the IDE240.DAT file in your openMSX roms directory. (e.g. C:\Program Files\openMSX 0.6.0\share\systemroms) Then start openMSX's catapult, and select a suitable MSX model (like the turboR GT) and the IDE extension. Put IDEFDISK.COM on your MSX-DOS2 disk image, and insert it in drive A. If the IDE extension is not in the list, either select
Test MSX Hardware from the File menu, or
Display Broken Configurations from the View menu.
Now start openMSX and wait for DOS2 to load. Start fdisk, either by typing IDEFDISK or by typing IDEFDISK /FAT16 if you also want to create FAT16 partitions. Using FAT16 has it's advantages and problems, and I won't cover it here. If you don't know how to make a FAT16 partition in FDISK, you probably shouldn't use one either.
If you chose to use FAT12 partitions, you'll be presented with the simple version of the FDISK tool. Just Press A when at the main menu to have it generate partitions automagically, then W to write the partitions to the virtual harddrive. Last press I to initialize (logical format) these partitions. You cannot specify the size or location of the harddisk by the way, it will always be 100MB, unless you edit the openMSX configuration files.
Reset openMSX, untill you've booted from the floppy again. Your floppy drive will now be called drive E: btw, since the first 4 partitions of your newly fdisk-ed harddrive are numbered A: through D: if you chose FAT12. Copy COMMAND2.COM and MSXDOS2.SYS to drive A: (first partition of the HD) and you should now have a bootable virtual MSX hd.
If you wanted to view the Dragon's Lair demo, then these last steps apply to you as well. First download and upzip the demo to an easy to remember directory, such as C:\TEMP\
Then start openMSX with the IDE device enabled. Once it's up and running go to the console menu which is usually under the F10 key. Then type the following:
diskmanipulator chdir hda1 <targetdir>
diskmanipulator import hda1 <sourcefile>
<targetdir> would be the destination directory on the MSX harddisk. This can be anything that actually exists on the first partition.
<sourcefile> is the path and filename of the file you want to copy from your PC's HD to the MSX's HD.
Remeber openMSX is made by quaint unox folx, so you'll have to use / instead of \ and you do not have to specify the sourcedrive. So the C:\TEMP\ from our example would be /TEMP/ instead. Unfortunately, you cannot use wildcards, so you'll have to do this file by file. After you're done, press F10 again to close the console, and check the contents of your virtual HD, your files should now be there, and you should be ready from some nice SC4 action.