[Content moved from http://photographingtransitofvenus.wordpress.com/]

Summary: Although you can probably use (a combination of) photographic neutral density (ND) filters to reduce the same amount of light as a typical solar filter (e.g. Baader solar filter), it is not SAFE for your eyes as it will not filter out the harmful Ultraviolet (UV) and infra-red (IR) light.

The table below shows the reduction in light intensity obtained by using different filters — both photographic ND filters of various Optical Density (OD) and the Baader solar filter. Just from the standpoint of intensity reduction, it is evident that one would require a 12-stop ND filter (OD 3.8) to match the Baader Photographic AstroSolar film. However, the following reasons undoubtedly makes the photographic Neutral Density filters unsuitable for solar photography: 

1. The photographic ND filters are designed to reduce light intensity effectively in the visible spectrum by absorbing the light energy in the visible wavelength. They may not filter out the harmful ultraviolet and infrared wavelengths. This can cause irreparable damage to your eyes if you use a ND filter to photograph the Sun, especially during a sunny day. The solar filters (for example, the Baader AstroSolar films and the Mylar filters/Mylar emergency thermal blankets) reflect back almost all of the light including light in near-infrared and ultraviolet region of the spectrum, making them safe for the eye. This point should be the reason enough for abandoning photographic ND filters for solar photography!
2. I am not sure how common it is to find a 12-stop ND filter, and even if you combine several ND filters to make an equivalent 12-stop ND filter, I am pretty sure it will turn out to be way more expensive than to use a proper solar filter.

Table:

Note on using a Mylar thermal blanket:
In my experiments, I have observed that a (single sheet of) Mylar thermal blanket is about 3 to 3.5 stops “faster” than the Baader AstroSolar photographic film. So, technically you can use a double-layered Mylar thermal blanket to photograph the Sun. However, there may be a loss of spatial resolution compared to the Baader AstroSolar film and remember that the thermal blanket was NOT DESIGNED for this! So please try to avoid using the Mylar thermal blanket if possible.

References:

1. “Solar Filters,” Society of Popular Astronomy
2. “Astrophotography Basics,” Kodak

[This content was originally posted by me at http://photographingtransitofvenus.wordpress.com/, a blog dedicated to the 2012 transit of Venus]