Ok, so looked up the exit tax on wiki but the wording is throwing me off:

The HEART Act, passed on 17 June 2008, created the new Section 877A, which imposed a substantially different expatriation tax from that of the earlier Section 877.[7] The new expatriation tax law, effective for calendar year 2009, defines "covered expatriates" as expatriates who have a net worth of $2 million, or a 5 year average income tax liability exceeding $139,000, to be adjusted for inflation, or who have not filed an IRS Form 8854[11] certifying they have complied with all federal tax obligations for the preceding 5 years. Notwithstanding the above, certain dual citizens by birth and certain minors as defined in Section 877A(g)(1)(B) are not considered "covered expatriates". Under the new expatriation tax law, "covered expatriates" are treated as if they had liquidated all of their assets on the date prior to their expatriation. Under this provision, the taxpayer's net gain is computed as if he or she had actually liquidated their assets. Net gain is the difference between the fair market value (theoretical selling price) and the taxpayer's cost basis (actual purchase price). Once net gain is calculated, any net gain greater than $600,000 will be taxed as income in that calendar year.

I'm confused if its the net worth of $2 million or 5 year average income of $139k adjusted for inflation or its the net worth over $600k.

Either way, it is a law to prevent people like Eduardo Saverin (co-founder of Facebook) from making huge amounts of money in the US from its resources and then bailing to avoid the tax bill. I for one would be happy to qualify for this tax (would mean I have lots of money).

But the interesting part is the exceptions to expatriate tax is:

became at birth a citizen of the United States and a citizen of another country and, as of the expatriation date, continues to be a citizen of, and is taxed as a resident of, such other country, and (II) has been a resident of the United States (as defined in section 7701 (b)(1)(A)(ii)) for not more than 10 taxable years during the 15-taxable year period ending with the taxable year during which the expatriation date occurs

So if you wife is American born and is paying UK taxes (which I'm guessing she isn't, since you are in the US and she is in the UK, I'm guessing she got her citizenship through marriage), she maybe exempted.