S Corporation Modernization Act of 2008 - Amends the Internal Revenue Code to revise the tax treatment of S corporations by: (1) reducing from 10 to seven years the period during which S corporation built-in gains are subject to tax; (2) repealing mandatory termination of S corporation elections for excessive passive investment income; (3) allowing S corporations to increase passive investment income from 25 to 60% without incurring additional tax; (4) allowing nonresident aliens to be potential current beneficiaries of an electing small business trust (ESBT); (5) allowing individual retirement accounts (IRAs) to be S corporation shareholders; and (6) allowing ESBTs to claim expanded charitable tax deductions.