The 560 Ti SLI is an outstanding choice, especially the factory overclocked versions which are being sold at or below the reference speed models.. Consider this, in Guru3D's game test suite, the GTX 580 scored a total of 616 fps.
By comparison, two 560 Ti's (900Mhz) gets 862 fps for $480 .... that's a 40% increase in fps for less money ! That 580 costing you $0.81 per frame whereas the twin 560's costing you $0.56 per frame. Here's how the most popular cars stack up .... total fps @ 1920 x 1200 (single card / SLI or CF) cost per frame single card - cost per frame SLI or CF
I'm a supporter of dual card setups to get the best performance (and better value).
560ti's in SLI provides a level of performance above what is available from any single card. As JackNaylorPE mentioned, the performance you get exceeds the GTX580 and you are paying less money for it. If you run into issues, things are complicated by using a dual card setup. However I would suggest the risk is well worth it.
If you are looking for a performance point below the most powerful single gpu card, then carefully consider if the savings is worth the potential headache of troubleshooting driver/config issues (I haven't had any problems but there are enough folks out there that want to pull their hair out working out kinks that it needs to be a factor).
This is quoted from this month's "Best Graphics for the Money" article from Tom's: "Two Radeon HD 6870 cards are a very powerful combination when paired in CrossFire mode, and the same can be said for a couple of GeForce GTX 560 Tis in SLI. Either of these options perform a little better than a single GeForce GTX 580 or Radeon HD 5970. But since the Radeon HD 6870 is notably cheaper right now, this option takes the recommendation."
I'm not advocating AMD over NVidia- I understand folks have preferences and that is really ok. Tom's recommended 6870's over 560ti's but I'd suggest a few extra dollars to get the brand you want at roughly the same performance level is more than ok. The fact that sli/crossfire setups take recommendations from Tom's and other sites at various price points indicates that potential issues should not be so prohibitive that you should steer clear at all costs. If you have a compatible motherboard, a sufficient PSU, and want that level of performance- go for it! Just understand that some folks have problems.
A pair of 560Tis will out perform any single GPU card available, also SLI is quite well supported so you don't need to worry about that, probably AMD provides more performance in CF than nVidia in SLI, but if you want to stay with nVidia you won't lose much.