Yes, you can.

If 'theTexture' is defined as a Sampler2D, you can do this:

half4 pixelColor = tex2D (theTexture, uv); // sample the texture at position 'uv'

and then read the r, g, b or a values using:

pixelColor.r
pixelColor.g
pixelColor.b
pixelColor.a

Read more about "Texture Samplers" on the nvidia cg tutorial, here: http://http.developer.nvidia.com/CgTutorial/cg_tutorial_chapter03.html

You can also use 'swizzles' to pull out custom combinations of components of the color array (or other 3 or 4 item vector), eg:

pixelColor.rgb  // would create a new half3
pixelColor.aaaa // same as half4(pixelColor.a,pixelColor.a,pixelColor.a,pixelColor.a)
uv.yx           // same as half2(u.x,v.x)

...etc

For more information on swizzling, read here (about one third the way down the page): http://http.developer.nvidia.com/CgTutorial/cg_tutorial_chapter05.html

For more on Unity, ShaderLab, Cg, GLSL and HLSL - please read:

Unity Answers - Can I use HLSL or GLSL shaders in Unity?

And, of course, the docs.