There's an interesting transcript here of a George R.R. Martin book signing in California hosted by Tad Williams. Before the signing itself started, Williams fired off some questions at George about his work. Particularly interesting is when Martin expands on how he was partly inspired to write A Game of Thrones after reading the first two volumes in Tad Williams' Memory, Sorrow and Thorn trilogy. Amusingly, Williams avoids responding to Martin's request to write more stories set in Osten Ard.
There's another report of the event and some pics here as well.