I also to run across this error when working with certain FreeBSD VMs (FreeBSD 7 and 8.0, 8.1). I think the vmware-guestd6 port is missing a dependency. Here is the problem:

# /usr/local/etc/rc.d/vmware-guestd start

/usr/local/etc/rc.d/vmware-guestd: WARNING: failed to start vmware_guestd

Not too helpful and the logs don't show anything either. If we try to manually start the daemon though we get something useful:

# vmware-guestd

/libexec/ld-elf.so.1: Shared object "libdnet.so.1" not found, required by "vmware-guestd"

The solution is to manually install libdnet which is the port that contains the above shared library.

# cd /usr/ports/net/libdnet
# make install clean

--- (snip of normal compiling) ---

The guestd should start without issue now:

# /usr/local/etc/rc.d/vmware-guestd start

Loading vmmemctl kernel module.
Loading vmxnet kernel module.
Starting vmware_guestd.

Note: This assumes you have the following in your /etc/rc.conf:

vmware_guest_vmmemctl_enable="YES"
vmware_guest_vmxnet_enable="YES"
vmware_guestd_enable="YES"