OK, now I know that the GA version of 5.1 is rapidly approaching. PBXT compiles with the latest release of MySQL without any changes!

This has never been the case before. Just search the PBXT code for MYSQL_VERSION_ID, and you will find things like:

#if MYSQL_VERSION_ID < 50114
    XT_RETURN_VOID;
#else
    XT_RETURN(0);
#endif

and, even worse:

#if MYSQL_VERSION_ID < 60000
#if MYSQL_VERSION_ID >= 50124
#define USE_CONST_SAVE
#endif
#else
#if MYSQL_VERSION_ID >= 60005
#define USE_CONST_SAVE
#endif
#endif

The lack of changes that affect pluggable storage engines can only mean that the bug fixes required are diminishing in scope.

And I believe this is a far better gauge of whether GA is close than any other …