Here's a development version of the x86-64 Linux client:
The official client for x64 Windows client can be found at boinc_5.10.13_windows_x86_64.exe.
The BOINC client 5.10 can now get 32-bit applications from projects that haven't added support for AMD64 (e.g., Lattice, SETI ß, etc), provided that they run at least the BOINC server 5.0.9. However, such AMD64 clients for Windows may not get applications from some projects that supported AMD64 before due to a platform name change, at least until such projects are updated.
For more information, see the BoincStats Forum.