Another option, with a bit of an early learning curve, but the effort will repay you handsomely, is to use one of the Dallas Semiconductor 1-Wire chips. That would give you 8 inputs for the "price" of one Arduino I/O bit.
1-Wire is often used by Arduino forum folk for the temperature sensors in the product line, but there are many, many other devices which work the same way. The 8 bit I/O chip is the DS2408, dta sheet....http://www.maxim-ic.com/datasheet/index.mvp/id/3818
I believe it can be configured to "catch" transient inputs, too... so you don't need to monitor the lines intensively.
More on 1-Wire generally at...http://www.arunet.co.uk/tkboyd/e1didx.htm