I've noticed a weird bug in ATV Black 1.5.1. From what I can tell, when the box goes to sleep while browsing the movies or while a movie is paused, on resume the SMB connectivity stops working. I've tried removing the share and adding a new one - aTV shows the network properly, adds a the network share but that doesn't work.
Resetting or restarting the device, doesn't help. The only solution that I found (from this forum) was to install XMBC, add an identical share and start a movie from there. This seems to somehow 'de-clog' the SMB connection which starts working w/o any other changes.
This happened to me twice already. I've changed the server name from host name to IP hoping that might address the problem. From what I can tell there's some weird caching going on that gets broken when the device shuts down.
I can confirm the Win share is working fine from XMBC and other machines - the appletv is wired and I can access youtube and the rest. It's just the SMB share, that used to work great, that for some reason stops working...