Can you make valid Makefiles without tab characters?

This is a syntax oddity/requirement of make, it has nothing to do with Mac OS X. Unfortunately, there's nothing you can do about it if you are going to use make.

Edit: GNU Make now supports a custom recipe prefix. See this answer.

You are not the first one to dislike this aspect of make. To quote Unix Haters' Handbook:

The problem with Dennis’s Makefile is that when he added the comment line, he inadvertently inserted a space before the tab character at the beginning of line 2. The tab character is a very important part of the syntax of Makefiles. All command lines (the lines beginning with cc in our example) must start with tabs. After he made his change, line 2 didn’t, hence the error.

“So what?” you ask, “What’s wrong with that?”

There is nothing wrong with it, by itself. It’s just that when you consider how other programming tools work in Unix, using tabs as part of the syntax is like one of those pungee stick traps in The Green Berets: the poor kid from Kansas is walking point in front of John Wayne and doesn’t see the trip wire. After all, there are no trip wires to watch out for in Kansas corn fields. WHAM!

In the time since this question was originally asked, a version of GNU Make has been released that allows you to use something other than Tab as the prefix character. From the mailing list announcement:

New special variable: .RECIPEPREFIX allows you to reset the recipe introduction character from the default (TAB) to something else. The first character of this variable value is the new recipe introduction character. If the variable is set to the empty string, TAB is used again. It can be set and reset at will; recipes will use the value active when they were first parsed. To detect this feature check the value of $(.RECIPEPREFIX).

This feature was added in GNU Make 3.82, released in July 2010 (six months after this question's original ask date). Since it has in turn been three years and change since that, it's likely that other Make flavors have followed GNU Make.