As was pointed out, RRSPs are afforded the same protection form tax in US as in Canada -- as long as you file Form 8891 every year.
When you do withdraw the funds you will have 25% (or 15% if a RRIF) Cdn non-resident withheld at source. It does not matter how little the RRSP is. If you have little or no other world income the year you make a withdrawal, you can submit a 217 return to get some or all of the withheld tax back.
Without getting into too much details about the US taxatuon of any RRSP withdrawal. the rule of thumb is that IRS will tax the growth from the period after you arrived in US.
Any other details should be sought at forums.serbinski.com