Since there's no limit on the number of items in an array as such, you shouldn't have any problem..
For the case when the array is embedded inside a document, you might want to have a look at these:
http://groups.google.com/group/mongodb-user/browse_thread/thread/4a7caeba972aa998?fwc=1
Filtering content based on words
http://groups.google.com/group/mongodb-user/browse_thread/thread/28ae76e5ad5fcfb5
