To chime in on this issue: in Montgomery County it is illegal to discriminate based on source of income. Piero-you are correct about the credit report-but only if the voucher is requiring the tenant to pay some of the rent. The landlord may require terms and security deposits that the tenant can not meet and the home must pass the Housing Opportunity Commission's standards to receive their funds. Other voucher programs may have inspections and requirements that can prohibit a tenant from moving in to a home as well.
This remains a "hot" issue and to keep informed of the latest will make you a student of the Landlord Tenant Affairs office in Rockville!