FedEx seems to be quite good at going offline for solid chunks of time on a fairly frequent basis.
Due to the corporate quarterly update, there will be a temporary service interruption from 9 p.m. to 12 a.m. CT on Saturday, Sept. 19, 2009. Customers using FedEx Web Services, FedEx Ship Manager® API and FedEx Ship Manager® API Direct solutions will experience interruptions with their transportation, revenue and rating transactions.
From 9 p.m. CT on Saturday, Sept. 19, 2009 to 3 a.m. Sunday, Sept. 20, 2009, customers using FedEx Web Services, FedEx Ship Manager® API and FedEx Ship Manager® API Direct solutions will experience interruptions with dispatch and scan postings.
As always, we recommend using multiple carriers just in case one of them errors out. Currently FoxyCart doesn't cache rates, so if FedEx is your only shipping carrier selected in your FoxyCart admin, rates may not show up and your customers may have trouble ordering. We do plan on improving shipping to cache rates so that something like this will fail more gracefully, but at present, if FedEx disappears for a few hours, so will their rate requests.