How are things on the homebrew front for DS?

As the NDS card slot is smaller than a CF slot and hardly larger than an SD slot, I doubt there ever will be a reader for the NDS slot. Those readers work rather like a drive than an adapter.

As for which reader you should take this is rather a matter of taste and price. Some are more comfortable and offer better music / movie playback other don't. Since you seem to be only interested in homebrew and emulation, you probably should go for the cheaper ones. Don't forget to get the right version of Passkey!

 

You only need the passkey once: Just so you can flash your DS and voilà you're done and you'll never need it again. Unless you're worried about voiding your warranty.
If so, you could use the G6Flash 4G which doesn't require Passkey and would again go into your GBA slot.
As for NDS slot equipped readers, I've never seen them so I can't comment on them.

 

I've got a mini-SD cardreader (actually, it's a bit more clever than that as it also has different types of onboard ram/flash for playing GBA games), and it's exactly the same size as a standard GBA cart.
Then use a PassMe2 to flash the firmware once, and you're done (with only the GBA cartridge sticking out of your DS Lite).

 

You should look online for a list of which ones are most compatible.
A card that went in the DS slot probably isn't impossible, but I haven't seen any yet.
The carts vary. Some just let you use SD card or compact flash, others are just flash ram in the shape of a cart, while others still are an actual gba cart (complete with battery backup and what not) with flash flash memory optimized for speed and not storage. Those are the ones that work best with playing roms, but are pitiful in storage size. If you're just interested in homebrew, I'd recommend getting one of the adapters so you can use a large SD card or compact flash. You can also use the WiFiMe program to just wirelessly upload files to the ds.

Oh, and I'd say FlashMe is a necessity. Much better than having to physically use a Passkey, I'm not sure if FlashMe is available for the DS Lite yet though. I think FlashMe requires you to either have a PassKey or WiFiMe though to do it the first time, but maybe they've figured out a way to do it without by now. (I'd choose WiFiMe over PassKey because it's free and you probably already have the hardware for it if you have a wireless card or laptop)

 

Flashme is available for DS Lite.

Just go with a M3 MiniSD or a Supercard MiniSD, you can't go wrong with them.

 

Note that the M3 (and also the EZFlash 4 which I have) contain additional memory-buffers (called PS-RAM and NOR-Flash I think) that increase battery lift compared to the Supercard.

 

Problem is that GudenOden is buying a DSL which has the latest firmware (v5/v6 I think) so WiFiMe is not an option as far as I know.

 

Supercard also has that additional memory (32 megabytes).

I have a supercard and i get 8 hours of gameplay with it, so I think it's pretty good, knowing that with original games you get 10 hours.