Voice and collaboration is where Nortel has a much better chance of remaining vialble. Nortel is already in the top four vendors, along with Avaya, Cisco, and Siemens and the market share difference is less than 10 points, that is where the Nortel Enterprise business has some legs. If that division could get some funding and some wins, they could remain a player in what is still a growth market. If an equipment vendor with an existing VoIP and UC product line gets ahold of Nortel's enterprise business, they are going to have to contend with supporting Nortel's gear while weaning them off what they have. Nortel customers are nothing if not loyal, so that could be a difficult move.
No matter who acquires the enterprise business unit, whether it is Avaya, MatlinPatterson, or some other bidder, Nortel jobs will be cut, product lines will be cut, and what remains will be leaner. I don't think Avaya will want to retain the switching and routing product lines. Avaya is pretty focused on voice and UC. Everything else is just a distraction. There is something to be said about doing a few things well rather than doing many things so-so.
If I were a Nortel enterprise customer, I'd be looking at options right about now. No matter what happens, you won't be dealing with the same company a year from now.