Based on ISC and Symantec's warnings below, it appears that MS08-021 is being actively exploited in the wild It is advised that folks apply the April updates as quickly as possible using the Windows Update process
QUOTE: The ThreatCon is currently at Level 2. The DeepSight honeynet has observed in-the-wild exploit attempts targeting a GDI vulnerability patched by Microsoft on April 8, 2008. The malicious image appears to target the Microsoft Windows GDI Stack Overflow Vulnerability (BID 28570).
At least three different sites are hosting the images; two different malicious binaries are associated with the attacks. Analysis of the images has shown that although they appear to be malicious, they do not contain enough data in the associated image property to sufficiently trigger the vulnerability.
We are still investigating as to why this may be the case. Users are advised to apply the MS08-021 patchesimmediately. These attack attempts highlight the severity of this issue -- it is only a matter of time before new images that successfully trigger the issue are observed in the wild.