That Kotaku article is really old; there are better post-release reviews of this.
The DSi is made to better compete with the PSP's in-firmware features, the IPod Touch's new game engine, and to better match the appearance of the Wii.
The cameras, as far as I can tell, don't serve much extra function. There probably are some new games that will incorporate them, but it will likely be minimal and social-based. The main improvement is the web browser built into the firmware, which still sucks compared to Bunjalloo, but is worth the price increase (as $DS Lite + $Opera Browser > $DSi). This use of wi-fi also opens up some more options, as Nintendo is allowing users to purchase and download additional games, software, and playable content, to be saved on an SD card. They removed the slot-2 functionality to make this work, so I'm not sure if the DSi still actually emulates a GBA, but I'm imagining Nintendo will sell downloads of GBA games to make up for this.
As far as homebrew goes - no slot-2 devices will work anymore, for obvious reasons. All slot-1 manufacturers have released NEW versions of their cards, so even though homebrew does still work on the DSi, the old cards apparently don't.
One thing this article fails to mention - the DSi's being sold in the US right now are Japanese systems, and since Nintendo is region-protecting games, it will only play Japanese releases. You also need to get a power converter in order to plug in the AC adapter. When NOA makes a DSi US release, I bet Gamestop, Gamecrazy, and other chains will do a trade-in deal like they did for the DS (GBA to DS) and DS Lite (DS to DS-Lite), where old system + $25-50 gets you new system. However, if the DSi is half as popular as the wii is, most of the buyers will be scalpers who are trying to artificially inflate demand.
Edit: To answer the WPA question; to the best of my knowledge, it still doesn't support WPA. Nintendo didn't do anything to improve client recognition on networks (which means I still couldn't go online at school), so I can't imagine they've improved their encryption protocol.