I have a laptop with dual boot, a windows 7 with encrypted partition and an Ubuntu 10.10. Now I understood that i can't mount encrypted partitions from Ubuntu, 'cause it's not a valid ntfs partition anymore. But i wonder whether i can boot this encrypted windows 7 from grub.
Yes you can. It is all explained very well here. A simpler alternative would be to stop using Bit Locker and start using TrueCrypt. Bit Locker is plenty secure, but TrueCrypt is way more versatile and offers many more options. Check out this review between the Bit Locker and TrueCrypt, very informative.