I have this theory that is probably crazy, but has gotten some support today with word of redesigns for the iPod line that seem to make them more like iPads, and another mention of a refreshed iPad (in addition to the mini). I think it's possible they are going to effectively retire the iPod line this fall, kill all but the touch and nano, and rename those remaining two models as iPads instead of iPods.
The impetus for major change is the death of the 30-pin connector. It supports the idea that the iPad will get redesigned to account for it, and Apple might possibly make a few other tweaks to improve performance. If they're looking to draw attention to their "all new line of iPads," they'll want everyone to perceive the top-line model as being brand new as well.
You can all but bank on them finally axing the iPod classic and, since it lacks a screen or iOS, probably the iPod shuffle along with it, realigning their lineup as follows:
iPad (current gen): $499
iPad (last gen): $399
iPad mini: $299
iPad touch: $199
iPad nano: $99
Another possibility is that Apple could begin to brand their iPads like televisions--by their screen size--in which case we'd have the iPad 8", the iPad 8" (previous gen), the iPad 6", the iPad 4", and the iPad 2". This would be a subtle nod to the forthcoming iTV, which presumably will come in at least two screen sizes and have similar branding.
The iPod line contributes almost nothing to Apple's sales at this point. Nobody buys iPods any more, except for the devoted. And that's not a good reason to continue the line, especially with Apple shifting into the TV business next year. Notice how iPods are not part of the Apple displays in stores like Best Buy. They're elsewhere in the store. (So are iPhones, admittedly, but for a good reason: need a carrier.) Converting them to iPads might spur new interest in these products, since everyone wants an iPad but not everyone can afford them. This might even be an effective hedge against the sea of major tablets coming out this year from Google, MS, and Amazon.
Also, Apple will not want to keep products with the old connector in the lineup longer than they have to be, which I think also supports a modified iPhone 4S with the new connector coming with the new iPhone this fall. That paves the way for removing the 30-pin connector from Apple's lineup entirely a year from now, and leaving it only on the iPhone 4 until then.
(In keeping with that timeline, I'm fairly sure the MagSafe 2 will have completely replaced the MagSafe 1 in a year, unless Apple holds off a little longer than that to replace the Thunderbolt Display with a Retina Display due to panel availability.)