As Microsoft has released Lync in RTM, documentation and evaluation are now available 
(PS: thanks to Damien Caro)
UPDATE By the way, Lync is already available for MSDN and Technet subcribers.
- Server evaluation: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=145A4AAA-1F0D-403A-AD10-A1166EC44C75
- Client evaluation (32 bits): http://www.microsoft.com/downloads/en/details.aspx?FamilyID=ac81c462-9a4e-4d0b-be7a-36ebebf43a1f
- Client evaluation (64 bits): http://www.microsoft.com/downloads/en/details.aspx?FamilyID=e9f07fed-2d33-476d-8e8f-68442b94f942
- Lync Attendee (user mode installation guide): http://www.microsoft.com/downloads/en/details.aspx?FamilyID=68a3ca04-a058-4e47-98ea-9e9af7ebd6e3
- Lync Attendee (administrator installation guide): http://www.microsoft.com/downloads/en/details.aspx?FamilyID=1772a5ad-9688-4861-8387-ec30411bf455
Group Chat :
- Group Chat Server evaluation: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=575484f9-2a93-4450-a4c7-7d08a1290700
- Group Chat client evaluation: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=4a202df0-c2ac-48b5-a621-1b3f22003632
- Group Chat administrator console: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=a50cf7a4-6e8c-48a3-8c54-2449106f1627
Documentation :
- Technet: http://technet.microsoft.com/library/gg293124.aspx
- Communication flux poster: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=ad8ff3fb-014e-4fd7-8003-436d896ab0c6
- Presence schema: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=b5732027-f3a8-4a48-af2e-4937e33f2481
- Protocole used by Office and Lync Server: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=e7a23d42-0835-440f-9400-badfe9672b21
- Tools and documentation to help Lync adoption: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=f01a17e2-bec0-42c7-b1ce-fd4842ab1c91