According to Dept of Labor it sounds like retroactivity is plan dependent, as in it ain't guaranteed.
Which seems reasonable because otherwise why would anyone ever opt in day one unless they had a pre existing condition.
You will get paperwork within 2 weeks so you may want to call the plan administrator and get that cleared up.
http://www.dol.gov/ebsa/faqs/faq-consumer-cobra.html
Q11: If I waive COBRA coverage during the election period, can I still get coverage at a later date?
If a qualified beneficiary waives COBRA coverage during the election period, he or she may revoke the waiver of coverage before the end of the election period. A beneficiary may then elect COBRA coverage. Then, the plan need only provide continuation coverage beginning on the date the waiver is revoked.