Latitudes and Inspirons share a common base design and components, but there are always a few key differences. Take the Latitude D600 and Inspiron 600m for example.
Both systems share the same basic chasis design, and it is clear from the external port placement on both machines that the motherboards are similar (if not identical) in design as well.
Dell aims the D600 at the corporate market, a market that demands a sturdier, work oriented machine. Because of these requirements, Dell makes the D600 with a stronger case (the LCD housing is magnesium alloy versus plastic on the 600m) and includes a trackpoint device in addition to a touchpad. The D600 is also limited to a 32 MB Radeon 9000 because most business users don't need gaming capability.
The 600m, in contrast, is aimed at the general consumer market, where price is more of a concern. Thus, the 600m uses cheaper plastic components in its chasis and lacks a trackpoint device. However, users of the 600m are more likely to want to play games, so Dell allows configuration of the 600m with a powerful 64 MB Radeon 9000.
In short, Latitudes and Inspirons are often very similar, with minor tweaks geared towards the needs of the target market.