Do you have an approximate date for when this might happen?

Only the IRS knows.   The IRS usually announces the start of the tax filing season (last year it was Jan 22, 2017) but they have not announced any 2018 date yet.   Hopefully it will be in late January and the form available by then.  (A few years ago when Congress make late year tax law changes, some forms were not ready until late March).

The tax law that affects the medical deduction was just signed into law (makes it 7.5% of AGI for everyone).  The official IRS Schedule A is still the 2016 version.  Until the IRS publishes the 2017 version with the changes and then issues the 2017 e-fileing specifications for Schedule A to tax software companies (like TurboTax) the form cannot be finalized.   Everyone is waiting for the IRS.

Why has TaxAct online made the change already?

You would have to ask TaxAct that question.   I believe TurboTax Online has been updated and the desktop will be soon.

The IRS has to approve TT's version of Schedule A first.

Click on Review and there will be a list of forms and schedules with the dates they will be available.

See this TurboTax support FAQ for IRS forms availability - <a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/1908854-irs-forms-availability-table-for-turbotax-individual-perso...>...

The IRS will not be accepting any type of 2017 tax return until January 29, 2018.  The IRS will hold all 2017 tax returns that have a tax refund which includes EIC and/or the Additional Child Tax Credit util mid-February.  They will release the tax refunds in late February if there are no errors on the tax return.

Right - even though TurboTax is going to update the calculations to use 7.5% AGI for everyone, the Schedule A will remain "Not for filing" pending the official IRS release and approval of the 2017 form.

Schedule A has now been updated in TurboTax version R12 and reflects the 7.5% of AGI floor.  However, Schedule A has still not yet been released for filing.