I will try to find it, but I've noticed even in my short period of watching Test cricket, the alarming drop in attendance in Indian Test matches. Even I remember watching full houses when any side of note came to a decent sized city. India-Australia Test attendance was a disgrace.
Unfortunately, grounds in India don't have websites where you can look this stuff up. But I'll see if I can find TV figures.
Dennis Lillee has commented (in his autobiography) that Test attendances in India had been relatively sparse - back in the early 2000's, before the advent of the 20/20 phenomenon.
Lillee did add that the match-fixing saga (which heavily involved India) alienated a lot of people. Would there still be a residual effect? Would people simply not turn up because they think that a match might be fixed, or as some sort of bizarre moral stand? Possibly, but it'd be less likely now than back then.
Is Test cricket dying? Well, it depends where you go. In England and Australia, it is still strong, but South African crowds (with the exception of maybe the initial post-apartheid) years have always shown a preference for ODI cricket. In New Zealand, cricket has always been overshadowed by rugby union, so no changes there. As for the West Indies, popularity seems to be strongly linked (I mean, even more than elsewhere) to the team's performances. For example, when they beat England, there was a lot of adulation and (seemingly premature) talk of a revival.
Is 20/20 affecting the popularity of Test cricket? Maybe, maybe not. If Lillee's comments are anything to go by, it may not be affecting it a great deal. I don't like 20/20 cricket, but it does have the potential to introduce a new audience into the game, which may then take a liking to Test cricket (even though Tests tend to be more idiosyncratic and about as accessible to many as a Nine Inch Nails album).
20/20 may rejuvenate Test cricket in another way - by demonstrating how marketing techniques can be used to make seemingly slight fare (let's face it, 20/20 lacks long-term memorability and any real sense of cricketing nous and substance) seem like something magnificent, then why not apply the same methods to a more subtle yet more complex form of the game and see where that takes you?
The only problem is that, since 20/20 has about as much in common with Test cricket as a piece of graffiti on one of the walls at Old Trafford, marketing techniques used to bolster 20/20 cricket's popularity may not work that effectively with Test cricket. I suppose this is where international strategic management (or some such) comes in, where you can market the same product differently in each country.
I know that other posters have touched on this, but without really noting the difference between the two formats or taking to account how marketing will have to adapt to the various cultures of countries.