I don't think open-sourcing an entire product is a good idea (at least on launch). However, open-sourcing components (such as your internal tools, file specs, libraries, etc) can add as much value to the open source ecosystem while preserving your competitive advantages.
In the case of crypto/security, open sourcing your algorithms and protocols can be a big win. You get free vetting of the security of your system. If you use LGPL or another suitable license, then you can still keep other parts of your product closed. Those are the parts that actually give your product "polish" and add the most value.
(I am considering doing this for a password vault app.)
When people will search for you online, you'd rather have them land on your blog, no? This is a great opportunity to market yourself. You don't have to blog to build an audience..
And it's not that hard, especially if you don't have a famous or extremely common name. I got to the top Google search result for my name in less than 3 months.
Another tip for accomplishing the same thing: being active on mailing lists and having a .signature with a link to your site. Open source projects are a great way to build up google points.
When you really need to get something done overnight, which is common in startups, you'll have to pay extra fees. And no contractor gets to the small details, or gets the whole picture.. Would you open a garage without being an engineer/mechanic? I doubt.
