The version is renamed:

MySQL Build Team · MySQL Build Team · commit 54df0057e18d · 2009-12-07T19:00:02.000+01:00
This is no tree that will progress through "beta" and "rc" to "GA",
rather it is a series of "milestones",
so the version is
                  5.5.0-m2
diff --git a/configure.in b/configure.in
@@ -10,7 +10,7 @@ AC_CANONICAL_SYSTEM
 #
 # When changing major version number please also check switch statement
 # in client/mysqlbinlog.cc:check_master_version().
-AM_INIT_AUTOMAKE(mysql, 5.5.0-beta)
+AM_INIT_AUTOMAKE(mysql, 5.5.0-m2)
 AM_CONFIG_HEADER([include/config.h:config.h.in])
 
 PROTOCOL_VERSION=10

Original file line number	Diff line number	Diff line change
`@@ -10,7 +10,7 @@ AC_CANONICAL_SYSTEM`
`10`	`10`	`#`
`11`	`11`	`# When changing major version number please also check switch statement`
`12`	`12`	`# in client/mysqlbinlog.cc:check_master_version().`
`13`		`-AM_INIT_AUTOMAKE(mysql, 5.5.0-beta)`
	`13`	`+AM_INIT_AUTOMAKE(mysql, 5.5.0-m2)`
`14`	`14`	`AM_CONFIG_HEADER([include/config.h:config.h.in])`
`15`	`15`
`16`	`16`	`PROTOCOL_VERSION=10`