I am (in the absence of my iBook) writing this on a 14" widescreen HP dv1224, a very nice machine that would be a very nice format for new iBooks (except for WInXP, etc). But a widescreen iBook would imply a complete redesign, something I would deem pretty unlikely before the introduction of Intel processors..
An aggressive Apple would go for the new widescreen form factor despite moving to an Intel board in the future. And if I read Jobs right, any high powered mobile chip will certainly be pegged for the Powerbook line.
As others have noted, a 14" widescreen iBook running at 1.5 gHz could cannabalize some PB sales, specifically the 12"...but perhaps for only a few months. September or October could bring new PB's at 1.8 or even 2.0 ghz G4's before the new MacTel PB's surface in mid 2006.
Pure speculation of course, but it's just that Apple needs compelling product offerings during this transition. A simple speed bump to 1.42 gHz may not be enough to get people to open their wallets.