The more I think about this, the more I'm convincing myself this acquisition is very likely to happen, at least for the GF1 US business (the Japan S/X factory maybe unlikely).
Panasonic has invested $1.6bn in GF1 capex. I guess it also has cumulative cash operating losses at GF1 of around $0.5bn? It may have invested some working capital too, let's say $0.2bn? (Tesla payables to Panasonic will be higher, but Panasonic has its own payables to suppliers). Then the remaining value in this business is the staff experience and manufacturing IP. The value of this part is very different for Tesla vs Panasonic. Assuming Panasonic knows Tesla plans to make cells for Model Y and future expansion in-house, it knows it is never going to get the scale to make its GF1 business significantly profitable - so discounted cash flow value of this IP is actually very low to Panasonic (unless they think they can sue Tesla for IP theft). For Tesla however, Panasonic's GF1 staff experience and parts of their manufacturing IP are likely very valuable (though parts of the IP will probably be substituted out with products from Tesla's R&D lab), particularly given the scale of its future ambitions and how much this acquisition could lower execution risk with cell ramp for Model Y.
Tesla looks in the stronger negotiating position here to me. Tesla will likely just go it alone with cell production if Panasonic don't sell up (possibly at risk of some lawsuits). If Tesla offers $2.5-3bn for Panasonic's GF1 business I can see Panasonic accepting (around 15% of Panasonic's market cap for a business with annualised EBIT at more than negative $200m with no obvious prospects for significant future cash generation).
If this is part of Tesla's plan, then they want to get the acquisition closed before they start to ramp their own new cell capacity for Model Y and before their March battery investor day. Given M&A can take time to close, then a deal announcement could be due very soon. Of course only if i'm not just going crazy with meritless speculation!