According to DS Fanboy, a whole bunch of Metal Slug games have been killed for the Gameboy Advance. The reason for this vile act is unknown unfortunately. The news originated from Famitsu, a pretty reliable Japanese magazine.
To be precise, Metal Slug, Metal Slug 2, Metal Slug 3 and Metal Slug X have all been cancelled. The weird thing is that these games were only supposed to come out on the GBA platform and not the DS which means a lot of work has been done for nothing. DS Fanboy thinks it might have something to do with the upcoming Metal Slug Anthology game which is supposed to come out next month on the Wii. A PSP version is also in the works but a release date is unknown at this point.
I’ve provided 2 screenshots of Metal Slug X which is included in the Wii version of Metal Slug Anthology to ease the pain.