I also think that this is a totally valid question, although it is in the wrong forum.
Kwyjibo - your comments about expecting people to have crystal balls are very strange. This is a rumors site, and someone asking "when do you think the TiBooks will be updated next, and what upgrades to you expect" should not be given a mouthfull.
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I think we will see an update before August, based on Apple's current TiBook upgrade cycle as pointed out above. As for what to expect, we may have a better indication in a few days. For example, if we see some new hardware with USB2 and FireWire 2 (not that I'm expecting that) then you can safely expect to see those technologies in the next TiBook update. You can also expect larger HDDs and faster processor speeds, because they are a natural part of hardware updates.
But I think you should go for it now. The current TiBooks are amazing machines, and if you have a use for it now, buy it and love it.