[Riim]

На C#

using System;
using System.Text;
using System.Text.RegularExpressions;

public class TextUtils
{
	// ....

	public static string Fading(string text)
	{
		if (text == null)
		{
			return text;
		}
		StringBuilder stringBuilder = new StringBuilder();
		for (int i = 0, length = text.Length; i < length; i++)
		{
			int opacity = (int)(100.0F / (length + 1) * (length - i));
			stringBuilder.Append("<span style=\"opacity: 0.");
			if (opacity < 10)
			{
				stringBuilder.Append("0").Append(opacity);
			}
			else
			{
				stringBuilder.Append(opacity % 10 == 0 ? opacity / 10 : opacity);
			}
			stringBuilder
				.Append("; filter: progid:DXImageTransform.Microsoft.Alpha(opacity=")
				.Append(opacity)
				.Append("); zoom: 1;\">")
				.Append(text[i])
				.Append("</span>");
		}
		return stringBuilder.ToString();
	}

	public static string InLength(string text, int length, int fadingLength)
	{
		if (text == null || text.Length <= length || length < 2 || length <= fadingLength || fadingLength < 1)
		{
			return text;
		}
		Match match = Regex.Match(text, "\\S{" + (length + 1) + "}", RegexOptions.Compiled);
		if (match.Success)
		{
			length += match.Index - fadingLength;
			return text.Substring(0, length) + Fading(text.Substring(length, fadingLength));
		}
		else
		{
			return text;
		}
	}

	// ....
}

Используется так:

Response.Write(TextUtils.InLength("01234567890123456789012345", 22, 10));

Варианта на javascript-е у меня нет, но думаю переделать не сложно.