Do you have wireless encryption enabled? If so, which type? WEP, WPA, WPA2? If it is enabled, try temporarily disabling wireless security to see if you can get a signal.
Also, check to make sure you have the latest firmware version for your router.
What channel are you using? Use only channel 1, 6, or 11 since these are non-overlapping. Download AirRadar or iStumbler to see how many other wireless SSIDs appear and which channels they're using. Move to the channel that's least crowded (probably 11).