I'm running Arch guest on VMxware fusion. When I use darcs (2.4.4) on a
projects inside a shared folder I get a error like that:
 % darcs check                                                                                                                                                                               
 The repository is consistent!
 darcs: mmap of '_darcs/index' failed: invalid argument (Invalid
argument)

If a copy the same project folder to a local drive (inside the VM) I
get it working.

I migth have the first guess to solve it.
Now I've installed the darcs 2.4.4 via cabal in an other guest with
Ubuntu 10.4, and it is working!
The VM integration in my Arch installation is quite problematic, I have
issues with video, mouse, ... and I think the reason is I am using the
package open-vm-tools while in the other VM (Ubuntu) I've installed the
vm-tools (non-free) from source.

Thanks for the bug report and extra information, Edgar.  Petr, is this 
something you might want to have a look at?  I've tentatively marked it 
not-our-bug due to Edgar's follow-up.  Maybe you should confirm (perhaps 
with more information from Edgar?)