For those who want to debug on 64-bit Windows under Visual Studio, we are pleased to inform you that this is now possible with nacl64-gdb used together with WinGDB. The detailed instructions are at http://www.chromium.org/nativeclient/how-tos/debugging-documentation/debug-nacl-application-with-wingdb
Give it a try!
wbr,
Evgeny Eltsin