In addition to the recently released Lync Cumulative Update 2 (earlier this month), there is another update for just the Lync Client.
You can download the update here. The corresponding knowledge base article (KB 2540951) is here.
This update brings the client version up to 4.0.7577.275 and addresses two issues:
2543532 Credential prompts occur when you sign in to Lync 2010 by using the Lync Online service 2543531 Lync 2010 does not authenticate after you resume the computer from hibernation