It is probably fair to say that the Windows 7 builds that have been floating around on the Internet are very stable and that only a handful of minor errors have been reported yet. According to Neowin the latest two builds of Windows 7 that leaked to the Internet may damage mp3 files under certain circumstances.
The problem was first recognized by users who noticed that the first few seconds of their mp3 files had been cut off. Some thought it was related to bad crossfading first but it soon turned out that the data was not there anymore. Many users thought it was caused by Windows Media Player 12 but it turned out to be a global problem.