Is GBA flash card compatible with the DS's GBA slot?

Yes some GBA flash carts work with the DS.
Heres a link:
http://www.gameboy-advance.net/nintendo_ds..._flash_card.htm

Thanks for the speedy response!

Something cool: If you have an E-reader, it will not fit in the original DS. However, it WILL fit in the GBA slot of a DS Lite!

yea my friend lend me a flash cart for GBA it has 256mbits and i put it in my ds and i selected GBa game and it loded it up flawlessly

This has been confirmed by countless sources. Please do not resurrect old posts.