As with all antiques and collectibles, Nihonto are frequently copied, replicated, and counterfeited.  In fact, there is a broad market for modern production and custom-made swords that closely replicate the Japanese sword of old.  One must be aware of the different kinds of Japanese and Japanese-style swords out there...

One must especially watch out for fake modern-made Nihonto that are being passed off as old and genuine.  At first, the differences between a genuine and fake Japanese sword may seem subtle and tricky; But as you gain more experience studying and viewing true Nihonto, it will become easy to identify and disregard fakes. 

Here are some guides that will help you discern the fake from the fabulous:

From the Nihonto Kanji Pages:
http://jssus.org/nkp/fake_japanese_swords.html

An eBay guide:
http://reviews.ebay.com/HOW-TO-SPOT-CHINESE-MADE-FAKE-JAPANESE-SWORD-JUNK_W0QQugidZ10000000002370903

Be aware that counterfeiters are always refining their craft.  So even if a sword seems real, make sure that you can trust the seller and have done your homework.  When in doubt, consult an expert.