My understanding is when you efile, the data is sent from your PC thru their servers to the IRS. But they do not permanently store your data locally on the servers. So there is no 'file' for people to hack.
Just for fun, Google search result
"If you use the TurboTax CD/Download desktop software, your tax data file is stored locally on your computer. When you e-file using the desktop version, the data temporarily passes through Intuit's servers to reach the IRS, but a permanent copy of your full tax return file is not stored online by Intuit after the transmission is complete. The only copy resides on your local machine and any backups you make. "
