Most of the 'current' systems I've looked at get sold pretty quickly and replaced with 'new' returned inventory. So, if your wondering if a certain machine will be cheaper tomorrow than today, probably not. Now, they do have sales in that department (and of course over time, equivalent units offered get offered at lower prices as the price of the real 'new' units trends down).
I've bought a number of machines from the refurb department, had one bad one (bad motherboard), but they came out next day and replaced it. Saved a good $1000 on that system too, so it was worth the hassle. Lately the deals haven't looked that good, but I still would look at them first when thinking of getting another dell.
Bill