Yes, the Summit x440 and x460 support 802.3at standard.
As per the 802.3at standard,
1) The PSE (switch) can support either “two-event classification” or “LLDP classification”
2) The PD (power device) must support both “two-event classification” and “LLDP (Layer 2) classification”
The Summit X440/460 models DO NOT support two-event classification. However, they do support LLDP classification which is compliant with the standard.