The first x86-64/linux snapshot has been uploaded.
As you can see from the regression test results, the compiler is very stable already:
http://www.freepascal.org/cgi-bin/testsuite.cgi?TESTOS=0&TESTCPU=7&TESTVERSION=0&TESTDATE= (i386 for comparison:
http://www.freepascal.org/cgi-bin/testsuite.cgi?TESTOS=0&TESTCPU=1&TESTVERSION=0&TESTDATE=)
However, most programs doesn't compile or work out of the box because of 32<->64 bit issues. You can find some information about these issues here:
http://www.freepascal.org/wiki/index.php/Writing_portable_code_regarding_the_processor_architecture
Also converted C headers cause often problems because of incorrect data type conversion so don't expect to install the compiler and compile a working 64 bit lazarus with it ;)