BootXChanger lets you change your Mac's boot logo

Unless you use an Intel-based Mac, in which case (if all the anecdotal user experience I've seen is any indication) you'll be lucky if you manage to fit a jaggedy two-colour image in there.

[rant] There used to be a wider range of modding options available before Intel Macs and Leopard. According to Unsanity, developer of the late Mac theming tool Shapeshifter (think WindowBlinds), Leopard uses a sort of hacked-together method of drawing its UI which is halfway between the old style, and the much more clean and modern CoreUI. This is why Shapeshifter only worked on Tiger and earlier systems. Unsanity seems to want to wait it out until Apple gets their act together and finishes their transition, and in the meantime, some decidedly less polished tools like Magnifique (which replaces system files, something Shapeshifter didn't do) and Façade (which I shouldn't call "less polished", because it won't even exist until at least a while after Snow Leopard's release, if it ever manages to escape its vapourware status) have come into play. UI theming was definitely the cornerstone of Mac modding, much like on the Windows side, and while it isn't dead, it's been, let's say, forced underground.

Things like boot-logo swapping are dependent on messing with core system files no matter how you slice it, so that's naturally susceptible to changes in the underlying structure and esoteric requirements of those files. On PowerPC, the images used had a fair bit more leeway: you could use more colours, and if I remember right, you could even use a different background colour. It just had to be the right dimensions and under a certain file size. Now, with Intel, the boot screen's background will always be the same grey no matter what, so your image needs to match, and most people's attempts at shoehorning more than a couple of colours (total) into the palette seem to've met with failure.

Knowing Apple, I can't imagine this progression away from customizability is entirely a coincidental side effect of the platform's evolution. They deserve more credit than they get for trusting their users to know what they're doing (Terminal.app wouldn't exist if they didn't, and there are lots of things about OS X that can be tweaked, scripted and reconfigured), but theming is one area where they tick me off. If Aqua is such a sacred cow they don't want being messed with, they should probably take a step back and examine their own schizophrenic treatment of it. HUD windows are all over the map. Progress bars haven't been updated since they were first designed for 10.0, making them completely mismatched with the entire rest of the UI. Every new app they release goes out of its way to light their own Human Interface Guidelines on fire and toss them into the nearest dumpster. And yet, through all of this, they do what they can to make life harder for anyone who wants to apply their own visual style to the UI — oh, no, couldn't have it being off-brand. It would be well within their ability to not only officially sanction theming, but make it a selling point and push it beyond what any other platform can do by allowing designers to use live Core Image effects (on the understanding that they could be disabled for better battery life in portables). But that's not how they roll, and I can't imagine it ever being so. [/rant]