[AT&T Samsung Galaxy S II Skyrocket SGH-I727] m4570d0n:
As a general guideline, you should not build for a device and use the nightly/official product property overrides if you are not a maintainer for that device. If you were to do this for CM, the build process would edit the build.prop and add ro.rommanager.developerid=cyanogenmodnightly or ro.rommanager.developerid=cyanogenmod. This allows you to download and install updates for official builds and nightly builds from ROM Manager. Since you are not a cyanogenmod maintainer, and builds they make after yours would show up as an update and users would be downloading official CM10 builds, not yours. You would also essentially be incorrectly giving CM credit for your build. That also means you are incorrectly attaching their name as the one responsible for any issues that could arise from something you did. Same goes for AOKP (but with goo.im, not ROM Manager).

That said, for informational purposes, you could add a new build type, like unofficial-nightly, or something like that. The common.mk file pulls info from the build.prop file found in /out/target/product/skyrocket/system/ which is defined, in part, from device/samsung/skyrocket/slim.mk. (or cm.mk if building CM10)

I'll break down the common.mk file and insert the variables for the skyrocket so that it makes sense. What this is doing is creating a conditional statement, just like using and IF/ELSE statement in MS Excel, and based on the situation, it will add these lines to build.prop which is where the build process will look to find the name to use for the final zip file created at the end of the build.

Definitions:

PRODUCT_VERSION_MAJOR = 2
PRODUCT_VERSION_MINOR = 8
PRODUCT_VERSION_MAINTENANCE = 0

PRODUCT_BUILD_PROP_OVERRIDES += BUILD_DISPLAY_ID=JZO54K

PRODUCT_PROPERTY_OVERRIDES += \
updateme.filter=$(PRODUCT_RELEASE_NAME) \
updateme.version=$(PRODUCT_VERSION_MAJOR).$(PRODUCT_VERSION_MINOR).$(PRODUCT_VERSION_MAINTENANCE)

So:

updateme.filter=i727 (see: slim.mk file)

updateme.version=2.8.0

ifdef SLIM_NIGHTLY
PRODUCT_PROPERTY_OVERRIDES += \
ro.slim.version=Slim-[COLOR=Red]$(PRODUCT_VERSION_MAJOR)[/COLOR].[COLOR=Blue]$(PRODUCT_VERSION_MINOR)[/COLOR].[COLOR=Orange]$(PRODUCT_VERSION_MAINTENANCE)[/COLOR][B]-NIGHTLY-[/B][COLOR=DarkOrchid]$(shell date +%m%d%Y)[/COLOR]

If you are making a nightly build, then:

ro.slim.version=Slim-2.8.0-NIGHTLY-10242012

else
ifdef SLIM_RELEASE
PRODUCT_PROPERTY_OVERRIDES += \
ro.slim.version=Slim-[COLOR=Red]$(PRODUCT_VERSION_MAJOR)[/COLOR].[COLOR=Blue]$(PRODUCT_VERSION_MINOR)[/COLOR].[COLOR=Orange]$(PRODUCT_VERSION_MAINTENANCE)[/COLOR][B]-OFFICIAL[/B]

If you're not making a nightly build then, if you're making an official build, then:

ro.slim.version=Slim-2.8.0-OFFICIAL

else
PRODUCT_PROPERTY_OVERRIDES += \
ro.slim.version=Slim-[COLOR=Red]$(PRODUCT_VERSION_MAJOR)[/COLOR].[COLOR=Blue]$(PRODUCT_VERSION_MINOR)[/COLOR].[COLOR=Orange]$(PRODUCT_VERSION_MAINTENANCE)[/COLOR][COLOR=DarkOrchid]$(PRODUCT_VERSION_DEVICE_SPECIFIC)[/COLOR][B]-UNOFFICIAL[/B]
endif
endif

Otherwise, you must be making an unofficial build, so

ro.slim.version=Slim-2.8.0-UNOFFICIAL

It's basically the same for the ro.modversion after that.

ifdef SLIM_NIGHTLY
PRODUCT_PROPERTY_OVERRIDES += \
ro.modversion=Slim-$(PRODUCT_RELEASE_NAME)-$(PRODUCT_VERSION_MAJOR).$(PRODUCT_VERSION_MINOR).$(PRODUCT_VERSION_MAINTENANCE)-NIGHTLY-$(shell date +%m%d%Y)
else
ifdef SLIM_RELEASE
PRODUCT_PROPERTY_OVERRIDES += \
ro.modversion=Slim-$(PRODUCT_RELEASE_NAME)-$(PRODUCT_VERSION_MAJOR).$(PRODUCT_VERSION_MINOR).$(PRODUCT_VERSION_MAINTENANCE)-OFFICIAL
else
PRODUCT_PROPERTY_OVERRIDES += \
ro.modversion=Slim-$(PRODUCT_VERSION_MAJOR).$(PRODUCT_VERSION_MINOR).$(PRODUCT_VERSION_MAINTENANCE)$(PRODUCT_VERSION_DEVICE_SPECIFIC)-$(PRODUCT_RELEASE_NAME)-UNOFFICIAL
endif
endif

If Slim nightly, then:

ro.modversion=Slim-i727-2.8.0-NIGHTLY-10242012

Otherwise,

if Slim Official, then:

ro.modversion=Slim-i727-2.8.0-OFFICIAL

otherwise:

ro.modversion=Slim-2.8.0-i727-UNOFFICIAL

You could just add the word NIGHTLY after the word UNOFFICIAL in those two conditional statements like:

ro.slim.version=Slim-$(PRODUCT_VERSION_MAJOR).$(PRODUCT_VERSION_MINOR).$(PRODUCT_VERSION_MAINTENANCE)$(PRODUCT_VERSION_DEVICE_SPECIFIC)-UNOFFICIAL_NIGHTLY-$(shell date +%m%d%Y)

ro.modversion=Slim-$(PRODUCT_VERSION_MAJOR).$(PRODUCT_VERSION_MINOR).$(PRODUCT_VERSION_MAINTENANCE)$(PRODUCT_VERSION_DEVICE_SPECIFIC)-$(PRODUCT_RELEASE_NAME)-UNOFFICIAL_NIGHTLY-$(shell date +%m%d%Y)

or you could insert another condition with the build you want to create.