It looks as though David Prowse is healthy enough to fly again and has several appearances lined up in the States in 2012.  He is scheduled to attend The Mad Monster Party March 23-25 in Charlotte, NC.  Tickets are $20 for a single day, $45 for all three and there's a $199 "RIP" (Really Intense Partyier) option with all kinds of perks.  Here's their website http://www.themadmonsterparty.com/index.html