If you guys really want to compare to PS3, there have been many comments about the differences. Very obviously, the Vita is less powerful than the PS3. But it's still really powerful.
Versus the PS3:
- you'll have to reduce shader use by about 50%
- CPU is certainly less than half of the Cell, but not as many games use Cell to the max
- MSAA is relatively cheap on the Vita vs PS3 (can be as little as 10% overhead, where PS3 is more like 25% for 2xMSAA, and 50% for 4xMSAA apparently)
- Vita has more main memory, but less graphics memory (512MB main, 128MB graphics), where you have to detract some memory of those 512MB for the OS, party chat and it's maximum of 5 concurrent 'small apps' that are allowed to run beside it. It seems though (from dev screenshots) that this is still leaves at least 400MB of main memory available for a game, and possibly more.
- both types of memory are most likely slower than PS3. I've seen a figure of 13GB, and it's not clear if this is VRAM, main RAM, or both but that VRAM is simply on a separate bus so that reading from both at once is faster than just reading from a shared pool (compare this to the iPad 3 that apparently has two busses to its single pool of memory to the GPU, to optimise that speed/throughput). PS3 is 22GB/s for both of its pools of memory.
- Polygons per (30fps) frame seems to be in the order of 200.000 according to Bend, at least for their engine. This can be up-to 1-3 million for PS3 games (where it should be noted that a really good game engine only needs to draw 1.2 polygons max for a 1280x720p image, or you'd have more polys than pixels, and that's per definition inefficient

(some actual GPU programmers are sure to disagree though

)
- Vita native resolution is 960x544 (522240) vs PS3 (for most games) 1280x720 (921600). This means that the Vita needs about 60% of the throughput of the PS3 games. It would therefore need to be (all pros and cons combined) at least 60% as efficient as the PS3 to be able to do similar things at native resolution.
- Vita uses components and APIs that are similar to modern PCs (and PS3s). Shaders work similarly, texturing works similarly, etc.
There's more information like this out there. The upshot is clear: the Vita is weaker, and most likely in most cases at least 50% weaker (and many areas considerably more so, others less or better) than the PS3 or more. It is allowed to be at least 40% weaker. As a result, some sacrifices need to be made in order to run ports.
Not all games use the maximum power of the PS3 however, so they can be ported quite well (something like Rayman origins or Virtua Tennis are good examples, and something like Virtua Tennis can then benefit from the MSAA advantage that the Vita has vs the PS3 for instance.
By profiling the games that have been ported, you can get an impression on what aspects are harder on the Vita and what things are easier. it is never clear cut however. If you compare the PS3 and the PS2, then relatively the PS2 had far more framebuffer bandwidth thanks to its very efficient VRAM (EDRAM), to the point that the PS3 can barely keep up with it at PS2 like resolutions for (old school) framebuffer effects, and it simply can't do them at HD resolutions. So even some PS2 ports to Vita are going to run into similar issues.
But overall, the Vita is way beyond the previous gen consoles. Feature wise it's 'current gen', and powerwise its somewhere inbetween the PS2 and PS3, depending on what feature you look at, but on average I'd say closer to PS3 than PS2, and programming wise obviously waaay closer to the PS3.