Anybody whose contract expires this year cannot be traded between the season's end (i.e., final game) and the end of the NBA fiscal year (i.e., June 30th). For Portland, that includes Memo Okur, Raymond Felton, Hasheem Thabeet, Jonny Flynn, Nicolas Batum, Craig Smith, and Joel Przybilla.
Jamal Crawford sort of counts, too, since he'd need to exercise his player option first before he could get traded during that time frame.
Last year, the reason Andre Miller could be traded during the June hustle and bustle -- with the NBA Draft being the main attraction during that time frame -- was due to his contract not technically being up. Rather than being expired, Miller's contract had a non-guaranteed salary for the upcoming season.
From Larry Coon:
“When the trading deadline has passed. Teams are free to make trades again once their season has ended, but cannot trade players whose contracts are ending or could end due to an option or ETO.”