I couldn't see anything in Tokyopop's press releases on their website about them picking up a Metroid manga, but according to people on the AnimeOnDVD forums, some retail outlets are aware of it, and say it's "coming soon". It's most likely just that Tokyopop licences so much stuff that they don't really bother telling anyone about most of them until it's close enough to the release date so that any hype generated won't wear off (unless they really *need* to announce it to stop people from fan-translating it).
Someone speculated that it's probably a Metroid manga that's been running in "Magazine V" since Prime and Fusion came out, dealing with Samus's early history (which seems similar to the web-manga version Josh mentioned), because Tokyopop has apparently been getting a lot of Magazine V's titles lately.
Other options would've been the old American comics that appeared in Nintendo Power when Super Metroid came out, or the new ones made for Nintendo Power by Dreamwave (the guys making the new Transformers Armada comics). And neither of those really seems likely (nor are they technically manga, but that's never stopped Stu Levy before).
Or there are the unknown myriad of smaller mangas you can sometimes encounter in Japan.
*RO points to his cute icon*