@Zoron: wrong. Windows XP will be able to do whatever it wants on a 7-formatted NTFS volume. In fact, even Windows 2000 can.
@sminlal: you are right, but vague.
To be more precise: Windows XP, Vista and 7 all use NTFS version 3.1. However, NTFS 3.1 is a highly complex and advanced file system, and in fact its design includes features that not all versions of Windows can support. So, there is no chance of Windows XP corrupting a Windows 7 volume (making all data impossible to access), however XP may destroy data that Windows 7 saved to disk, if said data is 7-specific (alternate data streams not copied, etc.)
If you're using NTFS for file storage, fear not: it'll work, you won't lose data, the volume won't be corrupted. I'd recommend against forcing the install of both Windows XP and Seven on the same partition, though (7 will create a 100 Mb boot partition for essential files; I'd recommend against touching it).