I think the market will go lower 2012, so I'd be fine with delaying a bit. On the other hand, things are still nicely down from all-time highs, so I wouldn't wait forever for ideal timing.
Think about something like a DCA with market level triggers. Maybe half when down another 5% from recent peaks and the rest when down 10%. Or thirds down to 15% if you think it will get that bad and you feel greedy. With a plan for investing the remainder if the market doesn't go low enough. I'm retired, so I just spend the extra cash over the next few years. You might want to have a backup DCA, such that if you haven't invested enough by a certain amount of time you just do it at that time. Make sure you are able to invest while the market is falling! Don't chicken out.
The official way to do it is to go ahead and toss it all in now. On average markets go up, so you're only losing money (on average) by staying out. Plus, if you already had your AA set up years ago, you wouldn't have all this cash lying around now. Buying now just sets the AA to what it would have been had you started earlier. This will average out OK if you do this a lot, but is scarier for a one time shot.
Make your best guess and don't be too disappointed when it isn't totally optimum. Just try to gain a little advantage.