To copy contacts from Nokia N900 to Windows Phone 7 follow the steps:
1) Export contacts as vcards on N900
2) Copy a folder with exported vcards to your computer
3) Merge all the vicards to one file; on Windows, run in a console:
C:\Users\user1\Desktop\Exported contacts>for %a in (*.vcf) do cat newline.txt >>%acopy /B *.vcf merged_contacts.vcf
4) Import the merged file on gmail, as contacts
5) Configure your gmail account on a Windows Phone to copy imported contacts