One comment on Dawn's answer ... During the course of the project, SPI = 1.00 doesn't necessarily mean that all work is done, as it can also mean that the work is being completed as scheduled (not ahead or behind). When all work is completed, though, the SPI will always equal 1 per Dawn's response (whether or not it came in ahead or behind schedule) since by definition the earned value will at that point equal the planned value for the project.
With regards to Roberto's question - Dawn is correct that the SPI < 1.00 is sufficient to say that the project is behind schedule, and for the purposes of questions you'll see on the PMP exam this is what you need to remember. That being said, in practice, Roberto is right that the critical path needs to be considered as well, as it is certainly possible that your project team is waaaaaay ahead on activities that are not on the critical path, but behind on critical path activities that will be difficult to get back on track, which could lead to an SPI > 1.00 but still run a high risk for finishing late. Similarly, you could be ahead of schedule on the critical path, but have SPI < 1.00 because you happened to schedule several non-critical activities earlier in the schedule that are now dragging down to the index.
So yes, in the real world you'll want to put the SPI into the broader context of what's happening (PMBOK Guide page 219), but again, when you see a PMP exam question that asks what the SPI value tells you, SPI < 1 means the project is behind schedule, and SPI > 1 means the project is ahead of schedule.
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One last comment - I don't mean to imply that you'll never need to go with the "not enough information" option on the exam. In particular, you'll need to be careful about questions that mix cost and schedule metrics. Sometimes you'll see a question about schedule where you're given cost metrics (either the CPI itself or the AC and EV values that allow you to calculate CPI), and you really don't have the information you need to answer a question about the project schedule (or vice versa). In these situations, just make sure to review the question carefully and determine what information you need to answer the question being asked, and then see if the question provides that information (or enough pieces that you can get calculate those values yourself).
