Is the RELENG_5_2 tag supposed to be usable yet?

I'm trying to update a 5.1-RELEASE-p9 to the current 5.2 RC via CVSUP.
For unrelated reasons I completely deleted the contents of /usr/src
and /usr/obj first.  I've CVSup'd RELENG_5_2 without problem; and the
buildworld works fine.  But the buildkernel stopps with (linebreaks
inserted for readability):

/usr/obj/usr/src/make.i386/make -V CFILES -V SYSTEM_CFILES -V GEN_CFILES
	-V GEN_M_CFILES |  MKDEP_CPP="cc -E" CC="cc" xargs mkdep -a
	-f .newdep -O -pipe  -Wall -Wredundant-decls -Wnested-externs
	-Wstrict-prototypes  -Wmissing-prototypes -Wpointer-arith
	-Winline -Wcast-qual  -fformat-extensions -std=c99  -nostdinc
	-I- -I. -I/usr/src/sys -I/usr/src/sys/contrib/dev/acpica
	-I/usr/src/sys/contrib/ipfilter -I/usr/src/sys/contrib/dev/ath
	-I/usr/src/sys/contrib/dev/ath/freebsd -I/usr/src/sys/contrib/ngatm
	-D_KERNEL -include opt_global.h -fno-common -finline-limit=15000
	-fno-strict-aliasing  -mno-align-long-strings
	-mpreferred-stack-boundary=2 -ffreestanding
/usr/src/sys/dev/usb/if_aue.c:104:23: miibus_if.h: No such file or directory
/usr/src/sys/dev/usb/if_axe.c:100:23: miibus_if.h: No such file or directory
/usr/src/sys/pci/if_dc.c:139:23: miibus_if.h: No such file or directory
mkdep: compile failed
*** Error code 1


There does not appear to be a 'miibus_if.h' anywhere in /usr/src.
Did it just miss being tagged, or is this a symptom of some other
problem?  Or am I premature in attempting to build from RELENG_5_2 ?



Thanks,
-Pat