On the days that I don't prepare dinner (usually weekends) we often end up eating later in the night, usually around 9:30 or 10:00.

How much of an impact does this make on progress?

For example: Today I did Interval X (from P90X+) at around 11:00AM, and I did Kenpo Cardio (also P90X+) at 6PM. Tonight we ate dinner at 10PM.

Is it safe to say that meals eaten late at night and far removed from a workout won't be burned as easily as other meals? How much of an impact will this have in the long run?

Ideally you want to quit eating about 2-3 hours before you go to sleep. Alot of people will say it doesn't matter, calories in calories out, but when you're sleeping your body isn't really burning alot of calories and quite simply, you'd rather your body focus on rebuilding your muscles than digestion.

In the end though if you just watch your calories you'll be ok.