I didn't think to try this when I was testing the Insteon starter kit.
Page 35 of the Insteon whitepaper (released after I returned the starter kit to Martin Custer at AutomatedOutlet.com) implies that Insteon receivers can 'hear' an Insteon signal even if there is an X-10 signal present at the same time.
If anyone has Insteon, a TM751 & an HR12A they can test this by holding down a Dim or Bright button on the HR12A while sending an Insteon command.