The Concise Oxford Dictionary: Significant: 1. having a meaning; indicative. 2. having an unstated or secret meaning; suggestive (refused it with a significant gesture) 3. noteworthy; important; consequential.
Up to now the accepted ConLib government's line (page 19) has been that any transfer of new powers would trigger a referendum, however it is worth noting this report from Gavin Hewitt in which he states (and presumably this came from Hague) that "Any proposal to significantly transfer new powers to Brussels would trigger a referendum." Note the introduction of the word 'significantly', although admittedly this is 'tacked' onto a section about the euro and its survival.
Note also that the Coalition's 'programme for government only promises that the block on any transfer of powers applies to this parliament only, a condition that also applies to membership of the euro. More worry though is the introduction of this word 'significantly' - who decides if any further 'power-grab' is significant? On thing you can be sure of - it won't be the British people!
Don't you just love 'representative' democracy?