I think the whole remodeled GBA SP thing is due to nintendo revealing their online plan, what will probably happen is the updated GBA will be an SP with wireless built in, rather than the attachment that the current one needs, in addition it'll have the ability to go online like the DS. This way those that don't want to shell out $150 can buy the cheaper "GBA SP online"version of the GBA.
In addition it'll extend the aging GBA a few more years, this way Nintendo will be able to keep from damaging DS sales while still milking the GBA franchise, at least until they are ready to release the real "Next Gameboy"