Frequency and channel matters.
Maybe a neighbor added a router, wireless phone, etc.
Most people use channel 1, 6 or 11 on their g frequency. Use a wifi survey software to monitor channel traffic traffic and pick the least used.
I remember having wifi issues a few years ago due to a Panasonic 2.4GHz wireless "broad spectrum" phone. Problem fixed by buying 5.8GHz phones.
Then a neighbor bought a wifi router...I changed my g channel and the problem was fixed.
Same issue at work...losing signal often...found out there were 12 different wifis in the area. Changed channel, issue fixed.
Expect a portable device like an iPad to be more sensitive to wifi traffic than a more powerful device such as a laptop.
Also, I find 802.11N to be more sensitive/picky than 802.11G, but that could be me.