Sounds like you may have "Automatic brightness adjustment" checked. I personally find when you have that enabled it auto-dims alot and gets to be annoying. I think the screen is so bright to begin with I disabled the auto brightness and keep it manually low (about 11%) all the time for indoors use. When I need to use it outdoors I hit the Brightness Toggle Widget I put on one of my Home pages and it jumps to 100% (use 100% sparingly; big battery hog!). Fortunately (or unfortunately depending on your view), I live in Michigan and we get our fair share of clouds so I don't have to use 100% brightness too much.
To disable auto-brightness, from a Home page hit the Menu key, then Settings>Sound & display>Brightness, and uncheck the box.
If you want a Brightness toggle widget, it comes with Beautiful Widgets, one of the most popular apps used by people around here. After you install it, long-press on a Home page and select Widgets. Then select Beautiful Toggle Widgets and you will see Brightness there along with some Power type stuff like WiFi, Bluetooth, etc.
Hope that helps!