The two programs mentioned above are good solutions prior to importing them into iPhoto.
If you have already imported them, you can select an image or set of images and use the Batch Change command under the Photos menu.
Select Photos/Batch Change...
Select Set "Date" to xxxx
Pick the date and time you want to set them too. You can pick a default increment of time for each photo.
I did a similar thing in that I shot a bunch of old family images dating back to the 20's, 30's, and 40's.
First thing I did was to batch change all to a specific date, well in the past, like 1/1/11. Then as I identified images, I batch changed the date to the correct date or as close as I could get it.
Generally I put the date to 1/1 of a given year if I know the year, but not the actual date. The "1/1" is my indicator that I don't know the real date. Or you can pick some other aribitrary day/month to be your "flag" that you don't know the real date.
For me, I scan a load in an evening, put it into iPhoto, and it may be weeks or months before I'm at a family get-together that allows me to date and add comments to some the pictures from other family members.
Jim