There are many caveats. The deal can be exercised either online or in-store. The description of the deal pretty much means you should line up, if you are going to try to grab one in a brick-and-mortar Microsoft store.
The deal, to be clear, is for Dell's Venue 8 Pro, a tablet which runs Windows 8.1 and comes with 32GB of Internal storage. It also, fortunately, has a microSD card reader (SD/SDHC/SDXC), as that limited amount of storage isn't much. This deal is the first in Microsoft's "12 days of deals promotion."
The online deal began at 7:00 a.m. PT. It can be found here, and the hammering of the website should tell you there is plenty of interest. Online, only the first 100 buyers will be able to get the $99 price. After that, the price rises to $199, and once the supply runs out, it rises still further to $299, back to the device's MSRP.
[Note: we finally managed to get through to the deal, and it's already expired online.]
The deal can also be accessed in store. There, however, only the first 20 customers have access to the $99 price. Fortunately, Microsoft stores don't open until 10 a.m., so those on the West Coast still have plenty of time to line up at Microsoft stores. The price then rises to $199 for the rest of the day -- until stock is depleted, at least. Reportedly, some stores may have only 10 units available that can be sold at the $199 price; others, though, will have more. It is unclear exactly how many units will be available at which store.
The Dell Venue 8 Pro is described as follows, in terms of its technical specs:
Intel Atom processor Z3740D with 32GB storage (2MB Cache, up to 1.8GHz)
Microsoft Office Home and Student 2013 Included (a $149 value)
8.0 inch IPS Display with HD (WXGA 1280 x 800) resolution with 10-pt capacitive touch
Dell Wireless 1538 Dual-Band 2x2 802.11n WiFi + Bluetooth 4.0
Integrated 1.2MP HD Webcam (front) / 5MP (back)
1 x Micro-AB USB2.0 (for trickle charging and data transfer)
1 x Headphone and microphone combojack
The Atom processor listed above is a version of this year's Bay Trail Atom processors. Last year's version was dubbed Clover Trail.