Apple has admitted that its iPad isn't very good at Wi-Fi, and suggests that customers having problems move nearer to their hotspot to get a signal.
Most iPad owners seem happy with the device's Wi-Fi performance, but signal strength is a problem, as is reconnecting to networks which have the same name but operate on a different frequency. Apple suggests renaming frequency-divergent networks and checking that the router has power for those who can't get a signal at all.
Wi-Fi has become increasingly complicated, offering a simplicity of use that belies the complexity of scanning various frequencies and selecting suitable networks to join. Many routers offer 802.11b/g, and many now offer 802.11n too with the option to run the latter at 5GHz instead of the increasing crowded 2.4GHz band. While that might just seem like backwards compatibility it actually means the router is running two independent networks which happen to share the same name, and that's what gets the iPad confused.
If that's your problem then Apple recommends renaming the networks, thus allowing the iPad to remember which is which. For users having other problems the advice is to check the router is switched on and consider moving closer to it.
Some iPhone users have long experienced problems with Wi-Fi, though the greater size of the iPad may help by allowing antennas to be more widely spaced. To be fair the majority seem perfectly happy with the connectivity they've achieved, at least until they get one-upped by a 3G-toting colleague sneering at their hotspot reliance. ®