Apple's iPhone apparantly also has problems with D-Link DIR-655 and DIR-825 access points; a possible solution might be found on this forum thread:

[Link: forums.dlink.com]

"Go to Advanced > Advanced Wireless and change beacon period for 2.4 Band to 75.
Change RTS Threshold to 2304, Change Fragmentation Threshold to 2304.
save settings. Done."

Also, when using WPA, double-check that you have enabled SSID broadcast. While a Pronto may connect to an access point which does not include the SSID in its beacon frames, when using WPA, the Pronto requires that the access point includes the SSID in the beacon frame.