I see two potential problems with going the green card route now as opposed to later;
1. She will not be able to move to London until the entire process is completed.
2. She will be liable to US tax on her worldwide income whether she lives in the US or not once she has the card.
It has now become very hard to lose permanent residency once you have it, you have to actually take active steps to renounce it. However, the INS may decide that card itself is invalid if you cease to live here. This could put her in the worst possible position; a US resident's tax liability on her worldwide income but no easy way of getting back into the country.
The rules changed in 2004.
There used to be a concept called 'abandonment' (actually this still exists) but now in practice LTRs are treated like US citizens who want to expatriate meaning that you may be subjected to tax penalties depending on your income and net worth

Post edited by: Politically Correct, at: 2007/11/03 16:30
Post edited by: Politically Correct, at: 2007/11/03 19:13