##C# - 128 characters
First try at Code Golf, and using C# seems to be a handicap here...
By using .Compute() you can use the string values and sum them directly. As a bonus this works for other operators aside from just "+".
Golfed:
using System;using System.Data;static void t(string a,string b){var e=a+"+"+b;Console.WriteLine(new DataTable().Compute(e,""));} Ungolfed:
using System; using System.Data; static void t(string a, string b) { var e = a+"+"+b; Console.WriteLine(new DataTable().Compute(e,"")); } Calling t("123","456"); gets you 579.