There is not a donation address for the whole development team; if there was, somebody would have to be in charge of keeping track of the bitcoins, deciding what they should be spent on, etc.
I don't want to be that somebody....
If you like the wallet encryption feature, send bitcoins to:
Matt Corallo : 1JBMattRztKDF2KRS3vhjJXA7h47NEsn2c
and Jeff Garzik : 1BrufViLKnSWtuWGkryPsKsxonV2NQ7Tcj
Matt (aka "BlueMatt" in IRC) did the hard work of making wallet encryption happen, and deserves a ton of credit for being persistent and reworking
Gregory Maxwell ('gmaxwell') also deserves credit and donations, he gave a lot of feedback and did a lot of testing:
gmaxwell : 1LjPAUKf23kDBy9sLJbiLfsvjde3ZdHcbJ
(corrected to give Jeff credit for the initial implementation-- sorry Jeff!)