If you're fine with your GTX260, then it's a very competant card, so unless you have that absolute itch to mess around with SLi, then wait and see if anything materializes, but be well aware that due to ongoing production problems, the GF100 product that does materialize (whether it' called the GTX4xx or some new naming system [which is the rumour I prefer]) it's likely going to be VERY expensive at first (likely over $500 even if the MSRP is set at $499) and by all accounts it's likely going to be the crippled 448 shader part, not the full 512 shader part which is what was demoed at CES. However, like any launch we migh be surprised by what they bring even in that part, and if you camp the e-tailers, you might be able to get one near your pricetag sometime near launch (unlikely to be as low as at MSRP for a while after launch if it is a good part, just like all other products it will likely launch, then shortage, then you see only the higher priced boutique models available for a premium or the etail price rises).
If there's no game you really need to play better than your current card plays, then waitm but also be aware, there will always be something 'new around the corner' which is why I mention the wait for... wait for... wait for.. above since there's always something about to come. Just look at the GTX280 launch, then it was wait for the HD4870, then wait for the HD4870X2, then wait for the GX2 version of the GTX280 (became the GTX295), then wait for the HD48xx refresh, etc.
I would say that waiting for Fermi makes sense to get a feel for nV's new architecture, but I wouldn't expect 'value' from that launch as you're likely going to be competing with pent-up nV fanboi demand for those who just want to buy something green that plays better than their GTX28x series cards since they'd never buy and ATi card, which is the same for the other side too who likely did the same with the HD5 series.
Another thing to consider is the Titles, like AvP coming out this week, and then other titles that might give better insight into the power of either architecture, even one Fermi chips launch it's unlikely we'll have an idea of their strengths and weaknesses for a few weeks afterwards, since the initial benchmarks will be primarily those nVidia controlled in their agreements with reviewers and thus little more than PR favourable tests, and only once everyday people have had a test to grind the retail cards in different situations will we get a better feel of overall performance of the two solutions.
For now if you're happy with your GTX260 in the games you play and the settings you like, then there's no reason to upgrade. Personally I'd find it far more interesting to play with the features of the HD5870 or Fermi than to simply monkey around with SLi.