I had this issue for the last few weeks on my Samsung Captivate. I've used the phone for over 6 months without a problem. I thought it was one of the WI-FI apps on the phone and removed all of them, but this did not resolve the problem. Only after upgrading to Android 2.2 did the problem get resolved.
Make sure you backup your phone before upgrading.
Follow the instructions at http://ars.samsung.com/customer/usa/jsp/faqs/faqs_view_us.jsp?SITE_ID=22&PG_ID=2&PROD_SUB_ID=55 7&PROD_ID=558&AT_ID=304008