I've been bitten by this too and second that it would be nice to get clear information about it (as far it's known).
I experienced data loss several times (VM continues to work, but lost previously saved data) and disk corruption with two VMs (guest failed to boot).
I did some research on the net and found the following informations:
The bug with async I/O and ext4 mentioned above seems to be https://bugzilla.kernel.org/show_bug.cgi?id=16165 and is not resolved at the moment. (Source: "Discuss the 3.2.4 release" in this forum)
The same source states that async I/O is a new feature implemented in VB3.2.
There is at least one other bug with ext4 async I/O, for files with more than 4GB, which is marked solved in 2.6.33.3 ("named ext4: fix async i/o writes beyond 4GB to a sparse file"), which means it's present in the Debian Squeeze and the Ubuntu Lucid standard kernels at the moment (at least I didn't found a notice about backports).
There are more bug reports and fixes for ext4, of which many are not yet in the Distro kernels (e.g. Ubuntu has this bug open: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/588069 to backport a bunch of them to the Lucid kernel), but I guess they are not as grave as the two above, since many people including me use ext4 successfully for more than a year (though my Virtualbox server has just been recently updated from ext3 to Debian Squeeze with ext4 and VB 3.2).
So it seems that it's indeed dangerous to use VirtualBox 3.2.x on ext4 systems until these ext4 bugs are fixed.
ext3 seems to be fine. I for one will revert to Virtualbox 3.1 for until that bugs are fixed, as I have to repair these machines anyway and I am not inclined to rebuild the whole server (which runs more than VB) just for this.
I have a guess that the data losses I experienced may have another cause, but I am not sure yet, so I'll open a seperate thread, if I get more evidence.
That's my knowledge about this problem at the moment.
I'll be happy to hear new information about, especially when it's solved of course.