Same issue and same work-around fixed issue for me too. I looked in it further in the Network preferences pane of both the macs and came to the following conclusion:
*On both the macs, WiFi is #1 under Set Service Order and Ethernet is at #2.
*iMac->Air: file from iMac to MacBook Air.
*Network Preferences pane - order of the network interfaces on left hand column.
Test Scenarios for AirDrop from an iMac to a MacBook Air, both on 10.8 and eligible for AirDrop:
1. iMac->Air (on iMac - WiFi with no IP, yellow dot, and above unplugged Ethernet, red dot) & (on Air - any order of Ethernet and WiFi): success 🙂
2. iMac->Air (on iMac - Ethernet with valid IP, green dot, and above WiFi with no IP, yellow dot) (on Air - any order of Ethernet and WiFi): no go 😟
CONCLUSION: For AirDrop to work with Ethernet connected and valid Ethernet IP Address, on the Network Preferences pane on the left-hand interface cloumn, we need to have WiFi above Ethernet. the only way to do this is to have a valid IP Address for Wifi and set the service order for WiFi priority, which can be done as follows:
::System Preferences -> Network -> unlock padlock -> click the gear (3rd button) below list of interfaces -> Set Service Order -> Drag Wifi to the top (or drag the one at top towards bottom till WiFi comes to the top) ->click "Ok" -> click "Apply".
BOTTOM LINE: WiFi needs to be #1 (above Ethernet) in the list of network interfaces for AirDrop to send a file with Ethernet connected and with an IP address. The receiving computer can have any order of network interface.
So, if the Ethernet has a valid IP Address, the only way WiFi will be above Ethernet is if there is a valid IP address for WiFi as well and the WiFi is set as the primary interface in Set Service Order option.
guys, let me know if I am not accurate.. 🙂