Hands-on experience is essential to any kind of forensics proficiency and knowledge. Whether you get that experience...
at work or in your own time elsewhere, you really can't pursue such credentials without spending lots of time learning and doing packet traces, analyses and forensic reconstructions of event sequences, attack signatures and so forth.
That said, you can pursue any number of programs in this area that will get you credentials in this subject matter, though it may sometimes be stated in terms of protocol analysis rather than forensics, per se. But the two topics are practically inseparable, so don't let this dissuade you from following any of the paths I'm about to recommend:
1. EnCase Certified Examiner
Requires six months of experience or 32 hours of classroom training in Guidance software's EnCase forensic analysis products (widely used by law enforce- ment and IT security professionals).
2. Pine Mountain Group Certified NetAnalyst program
Various levels of certification that include coverage of forensic tools and techniques throughout. No experience requirements jump out at me, but PMG really wants you to attend all four weeks (or more) of their training classes to get certified.
3. Sniffer Certified Professional program
Sniffer technologies wants you to have Sniffer and take their training, too.
4. WildPackets NAX or Network Analysis Certification
Same as above, except WildPackets prefers (but does not require) that you use Etherpeek and wants you to go to WildPackets Academy for training.
That's about all the directly relevant stuff I know of, but for a broader survey of the subject matter, visit www.informit.com and search on "tittel protocol analysis" to read an article I wrote for them that goes into more detail on this subject.
For more information on this topic, visit these other SearchSecurity.com resources:
News & Analysis: Veteran sleuth on the cutting edge of cybercrime investigation
Featured Topic: Computer forensics
Dig Deeper on Security Industry Certifications