I don't know about in 5.3.x, but in 5.2.x there are some int/long issues in the PHP code. even if you're on a 64-bit system and have a version of PHP compiled with 64-bit, there are several problems.
First, the code that converts post_max_size and others from ascii to integer stores the value in an int, so it converting "9G" and putting the result into this int will bork the value because 9G is a larger number than a 32-bit variable can hold.
But there are also several other areas of PHP code that are used with the Apache module, CGI, etc. that need to be changed from int to long.
So...for this to work, you need to edit the PHP code and compile it by hand (make sure you compile it as 64-bit). here's a link to a list of diffs:
http://www.archive.org/~tracey/downloads/patches/karmic-64bit-post-large-files.patch
Referenced from this php bug post: http://bugs.php.net/bug.php?id=44522
The file above is a diff on 5.2.10 code, but I just made the changes by hand to 5.2.17 code and i just uploaded a 3.4gb single file through apache/php (which hadn't worked before the change).
ope that helps.