No Opera for x86_64 GCC4/QT4? - Opera for UNIX - Linux, Solaris, FreeBSD - Opera Community
Found this forum topic, and it looks like QT4 64-bit builds are "just around the corner". The poster mentions they have some internal builds, but they've been experiencing some bugs which have delayed release.
There's indication that a 64-bit QT4 build will be available for the final 10.00 release.