Last year, Nintendo announced that the Nintendo DSi Shop would be closing down early this year. The last day for the DSi Shop will be March 31, and this FAQ answers questions about the process. After March 31, you won’t be able to buy or download software from the DSi Shop. Re-downloading it will eventually become unavailable as well, and Nintendo recommends you transfer your DSiWare to a 3DS while the transfer tool is still available.
Some DSiWare is also sold through the eShop, and that will remain available.