Trust me, we are just not sitting around here playing BINGO all day.
At this late stage in the game, rather than committting to an exact date for release, a vague progress update in a format that wouldn't give any advantage to your competitors would perhaps pay dividends.
Like many of the other folks that run businesses, I hate to lose customers and whilst appreciating that software development is akin to the length of an unknown piece of string, just a little more communication along the lines of "we have been working on clearer screenshots this week to make the manual easier to use" etc wouldn't go amiss. Anything that helps to retain your existing customer base whilst finalising the program has to be a good thing for Coffeecup.
Whilst losing the odd impatient customer through a protracted development cycle is inevitable, I'd hate to see that trickle turn into a flood after all the hard work you guys have put into this venture. Use the 'waiting' time constructively and stay in touch. That is all that folks are really asking.