I turn my wifi off at home every night, and when i switch it back on in the morning i cant be guaranteed my ipad will always pick it up first time. most times, but not all times.
Also, if my ipod is left un-touched for an hour or two, i find that too is enough to kick it off my system. dont know why, kinda like an idle mode, but it happens. It doesn't annoy me at all, just a minor pain in the ass.
Same as when i got over a friends house or my parents house where i've previously used their network - sometimes the ipad doesn't immediately play ball....so.....
....My tip to automatically log back in to any remembered network as quick as poss is to go into 'settings' mode, then switch 'airplane mode' on then off.
The device then quickly and automatically resets and gets back onto an even keel without having to manually search out your network again. very fast and works every time. :)