I have modded 20 or so Wii's with wiikeys (usa and pal) and i have never had ANY connect24 problems occur and they were all turned on [connect24] before I modded.
The only thing that messed up was the vote data, but you just have to erase the data and it works again.
well that topic on PSX scene forums in the wiikey section proves otherwise, it IS a issue, it's even mentioned in the Wiikey review on gba temp to turn it off! so i ain't bluffing.
and i quote
QUOTEI recommend that before installing the Wiikey you go into your Wii’s system settings and disable Wiiconnect24. Some users have reported Wiiconnect troubles after installation of a modchips. I disabled Wiiconnect24 and then re-enabled it after installation and I haven’t had any troubles with the Weather or News Channels. Also be aware that when you dismantle your Wii you will lose time and date settings (no game saves will be lost), be sure to re-enter the time and date immediately after installation.