Losing the firewire on an entry level iBook probably wouldn't be a deal breaker for people buying them. If Apple intends to release a much cheaper intel iBook with the lower feature set that an entry level Dell has, they could drop firewire, iLife (keep iTunes of course), airport, bluetooth, external video output, and the modem. Think "No-frills iBook". They could have this at a much lower pricepoint than their current entry level 12-inch iBook model and people would probably buy it. Higher end models could retain similar feature sets. This lower end iBook, while not popular with the current mac crowd, could entice new buyers to try macs, similar to the way the mac mini has. The iBook starts at $1249 (CDN), whereas the lowest end Dell model starts at $699 ($599 on sale at the moment). The Dell has an Intel® Celeron® M 360 (1.40 GHz/1MB Cache/400MHz FSB), 256 MiBs of ram (shared with video card), a 40 GB hard drive and XP Home. Many people wouldn't even get to this point however because they see the price tag of the Apple and the price tag of the Dell and choose the cheaper one immediately due to the large differential. After this they start to choose the feature set they want. No, not everybody operates like this but many do, and if Apple wants to grow their market share they can't keep all the hardware features they currently have in their base models.
Edit: Given some of Dells current screen offerings (1920x1200 in a 15" laptop) I would have a difficult time buying an Apple right now. That's not to say I'm leaping at the opportunity to buy a Dell but Full HD resolution in a 15" laptop isn't anything to shake a stick at either.