Ah, I found the problem! 8)
In the current (4/27/12), 64-bit version distribution there are two librxtxSerial files:
joefernandez@joefernandez:~/opt/arduino-1.0-64bit/lib$ ls -la lib*
-rw-r--r-- 1 joefernandez eng 182189 2011-11-28 17:05 librxtxSerial64.so
-rw-r--r-- 1 joefernandez eng 182189 2011-11-28 17:05 librxtxSerial.so
However, you can see from the file sizes that they exactly the same size (~177.9 KB). In the (working) 32-bit version they are different:
joefernandez@joefernandez:~/opt/arduino-1.0-32bit/lib$ ls -la lib*
-rw-r--r-- 1 joefernandez eng 182189 2011-11-28 16:56 librxtxSerial64.so
-rw-r--r-- 1 joefernandez eng 48304 2011-11-28 16:56 librxtxSerial.so
So I copied lib/librxtxSerial.so from the 32-bit distribution into the 64-bit /lib directory (overwriting the original) and now it works!
Looks like a mistake in the 64-bit linux distribution package. Please fix this, guys!
\Joe