I feel that the problem with TMG discontinuation is somewhat a technical issue, and now we have the official view from MS about it. I am more worried about the general "discontinuation" policy for such infrastructure products as TMG.
If a company builds its entire infrastructure around ISA and later TMG, it is pretty bad news to hear that the product is declared dead by the vendor.
Also there is a feeling that we (customers) are being betrayed or fooled in believing that some of the products in the list of software we bought from Microsoft in the Enterprise agreement is being discontinued in the middle of our agreement. If we knew that TMG will be discontinued in 2011, we would think twice about including it in our Software Assurance.
Who knows, maybe you will drop Exchange next year?