It's not currently possible to receive customised reminders for a Smart Lists, sorry -- reminder settings apply across all tasks in your account.

Korym, as much as I love RTM (which is a lot), given RTM's current reminder capabilities, I'd encourage you to look to Google Calendar for your birthday tracking needs. It has very flexible reminder capabilities, and the data can then be brought into iCal or Thunderbird. 30boxes is another similar site if you're not a fan of Google.

Plaxo and other contact managers do birthday reminders in advance...

Plaxo are very good at this. I use a mix of outlook, plaxo and rtm. It sound complicated - but it's not (promise)

A compromise would be to create a Smart list of birthdays where the tasks are invisible until one or two days before the date. Then, when they pop up, you'll know it's time to hit the shops. Once completed, the task will disappear from the list, so you'll only ever see those that are pending(unless you forget to check, of course!).This is nearly as good as a reminder, but requires that you check the birthday list on a regular basis(ie, daily! :-) ).
In general, I find RTM's reminders more than adequate to my needs.

There's no need to have a special Birthday smart list, just incorporate that search into your normal smart lists.

True. There is no need for a special Birthdays list. The fewer lists you can manage with, the better.