For anyone that hasn't already heard the sad news, an UPS airplane crashed earlier today. The pilot and co-pilot both died in the explosion. While the cause of the accident is being investigated, you might notice a delay in your packages. UPS hasn't stated that, but it's just to be expected in this type of incident.
UPS will release more facts about this accident as they become available, but the NTSB will be in charge of the investigation and will be the primary source of information going forward.
On ups.com, tracking numbers for impacted packages will show "Transportation Accident Occurred" as the exception code (Source: UPS Press Release)