I don't know of a single, authoritative place you could go to get this data. Things like this usually get thrown out in occaisional press releases. Still, Google reveals a few such press releases.
In 2006, PricewaterhouseCoopers predicted annual growth in the video games market of about 11% through to 2010:
http://www.gamasutra.com/php-bin/news_index.php?story=9793
In 2001, the IDSA (a trade organization which now appears to be defunct) said that the industry had grown 15% per year since 1997:
http://ign64.ign.com/articles/094/094061p1.html
In 2007, the ESA (new name for the IDSA?) reported that the industry had grown 17% per year between 2003 and 2006:
http://www.theesa.com/archives/2007/11/us_video_game_i.php
So, all-in-all, I'd say the industry is growing rather quickly, and rather steadily. This doesn't directly answer your question of whether more people are gaming, but I don't think you'd see growth like this without a broader consumer base.
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