Why is this hyperforeignism pluralisation perpetuated? Not only is it inconsistent - we don't use the Inuit pluralisation for "anorak" for a start - but it's also not necessary. The most correct plural form is "referendums" and this is reflected in legislation - see the Referendums (Scotland and Wales) Act 1997 and Political Parties, Elections and Referendums Act 2000. This is becoming one of those issues on which pedants are making a fuss in favour of the wrong form!
(P.S. My blog spellcheck still doesn't recognise "referenda" but has no problem with "referendums".)