What we've seen is that sometimes WMC is so busy when it first starts up that it doesn't reply to that message in a timely fashion. If we are unable to get the XML file, then the server won't show up for about 10 minutes (when we periodically look around to see if we missed anything).
Regarding your question about looking for servers when we come out of standby, we're actually awake-enough during standby to keep track of the comings and goings of servers, so there should be no need to do a new scan.
I have approximately 12,000 songs in 136GB and WMC starts up and finishes scanning a about 2-3 minutes on my measley P3 system. I consider this acceptable given that I never shut it down. It didn't always used to be this fast though. WMC used to take 15-20 minutes to start up and would thrash the processor at 100% usage! I discovered that the slowdown was due to playlists with bad and/or non-existent songs. It appears that WMC scans the entire shared folder(s) each and every time it runs across a playlist song that it cannot initially find in its library. I simply deleted the playlists and all is well speed-wise. You can easily tell if playlists point to songs that don't exist by reading the WMC error log.
I still have the problem others have described - many times the M1000 starts up and reports that the library cannot be found. There is no refresh command of any sort and the only "fix" I have found is to pull the power and start again. Another issue (related?) is that even if it does find the library it doesn't always work properly. For example, if I browse by artist it may work fine, but if I browse by album or song it will frequently state that no items were found. If I repetitvely browse by album it eventually loads correctly, ususally after the third attempt. I suspect a timeout issue, but where? The (wired) PC is a slow P3 with 256MB and a very fast Seagate Barracuda drive and the M1000 is wireless. I don't know the connect rate, but it is goes fast enough to allow streaming of WMA Lossless for hours on end without a hitch. I had the problem on two released firmwares and the second 2.1 beta