Typically, it takes about one week from when you payment is approved to get the secured express check. But now, it appears that publishers who opt for standard delivery have already received their checks. Typically, standard delivery can take one to two weeks longer than the secure express check. Hence the reason for concern.
Google uses DHL to ship those checks via secure express delivery. In fact, AdSense publishers are charged a $30.00 processing fee for each check delivered via that method.
Forum discussion at WebmasterWorld.