http://www.google.com/support/feedburner/bin/answer.py?answer=126303&cbid=-ygnkeihmmsts&src=cb&lev=answer
Will I lose all my subscribers in this process?
You should not lose any readers of your feed during this transition
process. All
feeds.feedburner.com URLs will redirect your readers to
feeds hosted by Google. Your readers will continue to receive your
feeds. However, the reporting and analytics may take up to a week to
adjust to internet traffic changes, so the number of subscribers you
see in analytics reports may be lower for a short period of time.
In addition, the algorithms we use to calculate subscribers and reach
at Google are more accurate and up to date than the algorithms
previously used at FeedBurner.com, so your numbers will not be exactly
the same before and after the transfer. We are constantly tuning our
analytics to ensure they are accurate and correct for our publishers,
but going forward we are only doing this within the Google
environment.