According to FierceWireless, you cannot just walk into your local Sprint store and pick one up though. You must qualify by having low reception issues. I'm not sure how they determine you have reception issues in your house, so it remains to be seen how easy it is to get one of these if you need it. If you do qualify though, the device is free, but you must return it if you drop Sprint as your carrier, so basically consider it a loaner.
AT&T rolled out their femtocell earlier this year, which they call their 3G Microcell. The problem is, they charge between $50 and $150 for the device depending on your plans, then they charge you $20 per month to have it turned on and on top of that, your voice and data through the device counts against your monthly allotments. Sprint charges your monthly allotments as well for voice, data and texts, but they don't charge you for the device or a monthly fee. This is how it should be. After all, you wouldn't need a femtocell if your carrier's signal strength was strong enough. The device is designed to make up for shortcomings in their network.
Be on the lookout for reviews if you think you'll qualify. There have been some AT&T users not very happy with their 3G Microcell. That is just pouring salt on the wound if you paid for the device and incur a monthly fee. Worst case scenario with Sprint's femtocell is it just doesn't work. You are not out any money. Best case scenario though is you can use your Sprint phone in the house without dropping calls or losing your internet connection during a download.
Correction: AT&T's 3G Microcell can be used monthly with no fee if desired, but the device will cost between $150 and $100 depending on whether you get a rebate for subscribing to AT&T's broadband service. If you subscribe to the unlimited calling plan for the 3G Microcell, you can get an additional $100 rebate, but will pay $20 per month for unlimited voice for domestic calls only. All of your data and text usage will still count against your plan.