Cap holds were essentially designed to prevent a team from undermining the salary cap by signing outside free agents before re-signing their own.
A hold cannot exceed the maximum contract offer allowed to a player on the open market, nor does it apply for restricted free agents once they are renounced.
But there is a set scale for players like David Lee and Nate Robinson, and you can study the parameters on Larry Coon's Salary Cap FAQ.
Both Lee and Nate are 4 year Larry Bird eligible, following the fourth season of their rookie scale contract meaning their cap holds would be 300% of their 2008-09 salary:
Lee- $2,682,049 x 300% = 8,046,147
Nate- $2,911,077 x 300% = 8,733,231
Meaning if both players are here next year, there is a heavier burden on the cap towards 2010, and for more on that check out Hahn's take.
More so than anything, I believe the holds are the main reason that the team hasn't been in a hurry bring either back. Sure, they could simply renounce and get nothing in return, which is why Donnie Walsh has to be devastated that there was no interested in a sign and trade for Nate.
I believe there may be one for Lee that can still be executed.