Justin de Vries, who blogs at www.allaboutestates.ca, has provided an excellent overview of recent law in this area. Click here to read his article.
The bottom line is that forensic handwriting analysis is acceptable to judges, but doesn't necessarily make or break the case. The courts seem to want to be able to make their own judgments about the validity of the handwriting. Mr. de Vries' article is interesting reading, so check it out.