I always have a problem with birthdays. I generally don't make a big deal about my own, and sometimes that leads me to forget my friends' big days. iCal helps me, and I have a few birthdays in there that have pop-ups a few days in advance. It works well, especially if I remember to make it a repeating alert from year to year.
Now, here's another solution that plays well with the iPhone or iPod touch. It's called Birthday Reminder [App Store link] and sells for $1.99US. It scours your contact list to pull birthday information, and shows you all the ones you have in a nice chronological list. You can also tap on anyone in the list and call or text them. It tells you when the birthday is coming up, and how old the person will be.
A couple of notes: First, I didn't know contacts even supported birthdays. You can add them on the iPhone or in the Mac app by selecting Add Field>Birthday. There are some other goodies in there too, like 'maiden name,' but that's a subject for another post.
After Birthday Reminder scans your contacts, you can set up the app to email you to jog your memory about upcoming birthdays. You can decide how many days in advance you want the warning, and you can also get sound effects with the notice, or not. You can't specify the time of day you get the warning; that's coming in version 1.1. As it is, they come at midnight, G.M.T.
At first, this app seemed a bit redundant, but as I used it I found it quite helpful and gave me a nice overview of birthdays I needed to worry about.
Here's some small nits... I don't think the GUI looks all that great. It is pretty plain, actually. I tried syncing the list to the developer's server for the email function. One day it worked fine. The next it kept crashing the app. You can't set multiple reminders. If you tell it to warn you 2 days in advance, that's all she wrote.
Birthday Reminder is a clever app. It's not life changing, but is certainly convenient to use. With a couple of the little bugs removed, and a few features added, I think it makes a nice, useful addition to your collection.