Municipal Bond Market Support Act of 2008 - Amends Internal Revenue Code provisions relating to the small issuer exemption from interest expense allocation rules for financial institutions to: (1) increase from $10 to $30 million the annual limit on small issuers of tax-exempt municipal bonds; (2) allow an inflation adjustment to such increased limit amount after 2009; (3) repeal aggregation rules relating to the determination of small issuer eligibility; (4) allow small issuers an election to treat borrowers separately for purposes of issuance limitations; and (5) allow financial institutions and brokers to hold up to 2% of their assets in tax-exempt securities without affecting their interest expense tax deduction.