3 of us did this in May last year after driving up and leaving the car in the Old Blair car park. We started at about 2.00 pm and followed the old Right of Way (grassy track) high above the East side of the valley, and joined the main estate track (not tarmacked) a couple of miles South East of Forest Lodge. We camped on a huge area of soft grass about a mile past the lodge. The good land rover track continues well up the glen before climbing over towards the Tarff bothy area but the path right through to Bynack Lodge ruins is a good one. There's loads of good camping spots if you are not focussed on the obviously well-used areas of green, but hard ground.
We walked in dry weather, so crossing the Bynack Burn and the Geldie was acheived without taking boots off, but a party of 4 Germans following us walked up and down for ages before crossing - we didn't wait to see how they managed it.
From there to White Bridge is land rover track again and we found an excellent camp spot for our 3 tents right next to the river on an area of deep, soft dead grass which made for a comfortable night, there was even a rock platform for cooking on. Again, there's plenty of camping spots right up the lower part of the Lairig - all you need is a water supply and a soft, flat area big enough to take a tent.
There are areas near the Pools of Dee where tents have been pitched, but we had a 3rd night next to the burn in Rothiemuchus forest, between Picadilly and the Cairngorm Club Foodbridge, and walked into Aviemore to catch the train the next morning (about 3 miles).
Here's a link to my report - it shows camping possibilities and the Geldie crossing.
http://www.walkhighlands.co.uk/Forum/viewtopic.php?f=9&t=12572