People on this forum seem to be pretty damn confused about DS homebrew.
I was (and still am) an active member of several DS homebrew communities, so allow me to set you all straight.
First off: NEVER buy an R4 or M3 Simply. These are popular, but they are all actually fake. The R4 (and the M3 Simply, which is just an official clone of it) was once a good cart. Not anymore. They are outdated (terrible interface, terrible physical design), over-priced and, amazingly, no longer in production. The R4's you see are all actually clones and copies made by other companies. Any R4 or M3 you find are actually copies, as are the supposedly new and upgraded M4, M5, N5, N6, R5, R4 SDHC, R4i, R4i Gold, the other R4 SDHC, R4i SDHC, R4 Upgrade 1, 2 and 3, G6, G6 Gold (and the list goes on) are all crappy fakes made by other companies. Now, these may play some games and some homebrew, but they quickly lose support. And then you have a cheap, crappy cart that can't play any recent games or good homebrew. As a general rule: ANYTHING R4 IS FAKE AND SUCKS. The N5 actually broke a lot of DSs with its shitty wiring.
The only reason people buy them is because they are famous and they hear about them from friends. And if you go online and ask for help with one, you will be branded a noob and told to get a new, non-R4 cart.
Instead, get either an AceKard 2i (and install the top-of-the-line AKAIO unofficial firmware, which really makes this cart excellent), a M3i Zero (which isn't a fake of an R4 and is pretty damn good, if expensive), an EDGE cart, an E-Z flash Vi (frequent updates to its firmware, which is based on the beautiful Moonshell 2), the Supercard DSONE (often overlooked) or the soon-to-be-released Supercard DSTWO (which will be the first cart ever to play GBA games using only a Slot-1 cart, ie: the only way to play GBA games on a DSi).
Second off: Although the DSi has a lot of new hardware and features, these are all locked off and scaled down unless a DSi enhanced game is inserted. This means that the DSi runs in DS-mode (which is identical to an orig. DS or DS lite) whenever you play a normal DS game. While it is true that we can't access DSi mode yet (meaning that DSi games can't be pirated with the added DSi-part there), DS mode is completely usable. What this means is that you can use any DSi flashcart (it has to be a DSi-flashcart, but they cost the same as a non-DSi flashcart and are backwards compatible anyway) and the DSi will just act as a regular DS.
Now, there are two major problems with DSi flashcarts: DSi firmware updates and Wifi homebrew. It is true that Nintendo make their firmware updates manditory and ship all new DSis with the latest firmware, however- there has only ever been one firmware update which tried to block Flashcarts (which worked by checking the cart's heading to see if it was a forbidden cart) and this has been bypassed by all the flashcarts listed above. All you have to do is update the flashcarts bootloader with another DS (or the DSi if it hasn't been updated yet) and it will work with any DSi firmware so far released. The other problem is wifi. Many homebrew games that use wifi (which is literally about 1/2 the speed of dial-up on the DS anyway) will not run. There is no known solution to this yet, but it does not affect many pieces of homebrew.
Sorry for the rant, but I figure you all needed an update on the DS-scene.