After upgrading to the latest 4.0b4pre from 3.6.9pre (this could be in all 4.x though), I've noticed that the spellchecker is not enabled by default as it was prior to the upgrade. Altering the layout.spellcheckDefault from 1 to 0 then back to 1 didn't make any difference (tried from the options dialog too, by enabling/disabling the 'Check my spelling as I type' checkbox). Now, one has to manually right-click in each text box in every tab to enable spell checking. What a time waster!
Assuming you mean Wednesday's nightly then this should be fixed in today's (in a little while) by the backout of bug 581576