Dave,
The “correct” definition of measured SL should be driven by overall business objectives and, specifically, the business importance of abandoned calls. Call centers that contribute to sales, either directly or through upselling and cross-selling, should treat any abandoned call as a missed opportunity, and incorporate the number of abandoned calls in the SL calculation as per the first approach you proposed.
Conversely, if you provide unique a service and callers that abandon will eventually call back, you can use the second formula.
There are additional variations of these formulae that look more closely at the type of abandonment: short abandons, abandon during the VRU message and so forth, allowing you to better reflect both customer expectations and business objectives.
As Kala noted, different ACDs may use different approaches and it is important to ensure that the SL they report matches your strategy and the target SL you set for your operation! Some ACDS allow you to select from a list of several SL calculation algorithms.
You can read more about the business perspective of abandoned calls in
http://www.diagnosticstrategies.com/abandonment.htm and about SL calculations in
http://www.diagnosticstrategies.com/service_level.htm (an updated version will be posted early next week)
Hope this helps.
Joe