PartnerNet, an alternate form of Xbox Live for developers to use for testing code, according to a poster at the SHMUPS forums. The game was reportedly a 49.04 MB download, and captured screenshots show menus for the leaderboard and replay system. Also noteworthy is that the game supports both 16:9 and 16:10 aspect ratios.
Ikaruga's appearance on PartnerNet does not necessarily mean the game will be released anytime soon, although there does seem to be an indication that the game is nearing completion. Ikaruga was announced for XBLA alongside Rez, which is slated for release this month.
[Via Arcade Renaissance]