if you can get five people to buy CDs, and maybe two of them get another two or three to buy CDs, you'll have done much more good for the project than cutting a check.
I agree that the CDs are becoming an anachronism, but they still provide an easy way to contribute to the project. Buy the CD, throw it away if you don't need it.
Are there any strange tax or accounting advantages for the organisation I'm supporting if I buy actual stuff rather than donating?
ps. Donations by users are kinda hard to help (in term of numbers) but donations from companies, should be really waaaaaay more towards all BSD project but ESPECIALLY towards OpenBSD for reasons already mentioned.