Same for me having a DSi and BEUP 0.5 doesn't work. I think it's router's problem and not the DSi's.
First. Dsi can't connect automatically to my router (brand Baudtec) from the 3 first wifi settings (slots 1,2,3). I get error code 52001 -2. But I can connect automatically from the advanced 3 additional wifi settings (slots 4,5,6)...but i think homebrew's doesn't use them. I tried a DSL and it connected automatically to the same router...So first of all there is something weird with the new DSi's wifi. When i say "connect automatically" i mean just choose SSID and then DHCP configure the rest.
Second I used manual settings for the first wifi slot but i set as DNS IP, the router's address. Again error 52001 -2.This works on a PC but not for the DSi.
Finally I set as DNS IP, my ISP's DNS IP and finally DSi connected, but still BEUP doesn't work. Maybe beup also use gateway ip address as a DNS address also.