I am on the Nets e-mail list and they just sent me this:

During the course of the normal physical examination taken by forward Shareef Abdur-Rahim on Wednesday, August 3, 2005, to complete the trade, a question arose concerning a portion of the testing, Nets President Rod Thorn announced today. The Nets are currently in the process of seeking other medical opinions, and until further notice, the trade with Portland for Abdur-Rahim has been put on hold. The Nets have until August 12, 2005, to complete the trade.

As a result of this development, the Nets have cancelled today's scheduled press conference to announce the team's free agent signings.
This isn't good news, hopefully everything turns out ok or there goes the great offseason.