One more suggestion, based on some recent forum topics -- Make SURE that AS isn't going to have any trouble logging in to your ebay account BEFORE the snipe is due.
If you set up your snipe by just typing the auction number in manually, instead of importing it, AS never had to verify your user name and password, and if you've entered them wrong you might not find out until AS tries to place your snipe and discovers it can't log in for you...
Also, if someone has been trying to hack in to your ebay account, or trying to access it with an older password, ebay may set up a second-level security question that you'll have to answer before it will let you log in. But AS won't know that, and won't be able to log in.
To check this, first make sure you've got some items in your eBay "Watch List". Then go to My Snipes and click the "Import Wizard". If AS can log in to your account, the AS Import Wizard will show you the items in your eBay Watch list. So you're set. You can either use this screen to continue importing the items you want to snipe, or cancel it and rest easy...
If for some reason you CAN'T import, then check two ways. First, go to MySnipes, click Modify Account, and reenter your ebay user name and
ebay password. I don't know if these are case sensitive, but enter them EXACTLy as you enter them at eBay. Then log in to ebay as you normally would, and make sure ebay isn't asking for any extra info.