Add spaces before Capital Letters

Didn't test performance, but here in one line with linq:

var val = "ThisIsAStringToTest";
val = string.Concat(val.Select(x => Char.IsUpper(x) ? " " + x : x.ToString())).TrimStart(' ');

The regexes will work fine (I even voted up Martin Browns answer), but they are expensive (and personally I find any pattern longer than a couple of characters prohibitively obtuse)

This function

string AddSpacesToSentence(string text, bool preserveAcronyms)
{
        if (string.IsNullOrWhiteSpace(text))
           return string.Empty;
        StringBuilder newText = new StringBuilder(text.Length * 2);
        newText.Append(text[0]);
        for (int i = 1; i < text.Length; i++)
        {
            if (char.IsUpper(text[i]))
                if ((text[i - 1] != ' ' && !char.IsUpper(text[i - 1])) ||
                    (preserveAcronyms && char.IsUpper(text[i - 1]) && 
                     i < text.Length - 1 && !char.IsUpper(text[i + 1])))
                    newText.Append(' ');
            newText.Append(text[i]);
        }
        return newText.ToString();
}

Will do it 100,000 times in 2,968,750 ticks, the regex will take 25,000,000 ticks (and thats with the regex compiled).

It's better, for a given value of better (i.e. faster) however it's more code to maintain. "Better" is often compromise of competing requirements.

Hope this helps :)

Update
It's a good long while since I looked at this, and I just realised the timings haven't been updated since the code changed (it only changed a little).

On a string with 'Abbbbbbbbb' repeated 100 times (i.e. 1,000 bytes), a run of 100,000 conversions takes the hand coded function 4,517,177 ticks, and the Regex below takes 59,435,719 making the Hand coded function run in 7.6% of the time it takes the Regex.

Update 2 Will it take Acronyms into account? It will now! The logic of the if statment is fairly obscure, as you can see expanding it to this ...

if (char.IsUpper(text[i]))
    if (char.IsUpper(text[i - 1]))
        if (preserveAcronyms && i < text.Length - 1 && !char.IsUpper(text[i + 1]))
            newText.Append(' ');
        else ;
    else if (text[i - 1] != ' ')
        newText.Append(' ');

... doesn't help at all!

Here's the original simple method that doesn't worry about Acronyms

string AddSpacesToSentence(string text)
{
        if (string.IsNullOrWhiteSpace(text))
           return "";
        StringBuilder newText = new StringBuilder(text.Length * 2);
        newText.Append(text[0]);
        for (int i = 1; i < text.Length; i++)
        {
            if (char.IsUpper(text[i]) && text[i - 1] != ' ')
                newText.Append(' ');
            newText.Append(text[i]);
        }
        return newText.ToString();
}

Your solution has an issue in that it puts a space before the first letter T so you get

" This String..." instead of "This String..."

To get around this look for the lower case letter preceding it as well and then insert the space in the middle:

newValue = Regex.Replace(value, "([a-z])([A-Z])", "$1 $2");

Edit 1:

If you use @"(\p{Ll})(\p{Lu})" it will pick up accented characters as well.

Edit 2:

If your strings can contain acronyms you may want to use this:

newValue = Regex.Replace(value, @"((?<=\p{Ll})\p{Lu})|((?!\A)\p{Lu}(?>\p{Ll}))", " $0");

So "DriveIsSCSICompatible" becomes "Drive Is SCSI Compatible"