I had this bug with the choose connection dailog box popping up all the time last night because I had set my n900 "search interval" in the internet connections settings to be Never - I changed this while Yappari was running. After restarting Yappari it was ok again.

- Note, I normally have my n900 set to search automatically for wifi every 30 mins, I only disable that when my home internet is broken so I need to stay on 3g

As for my changing between 3g and wifi I haven't installed anything extra to do that, it is built into the n900 in the internet connections settings. When it detects a saved wifi connection it seems to bring up the wifi and then just switch to that as the default route before disconnecting the 3g.