They have invested pretty heavily in facilities/equipment, growing from $2.8B to $7.2B over the past four years. I'd imagine that's stuff like their giant new data center and partnership agreements in their Asian operations. Where they really bulked up is "Long Term Investments". None four years ago, $25B today (and just $10B last year). Unsure what that would entail...
The relevant thing is how much is liquid fast enough to use it for what they want to use it for. For that we should listen to Apple telling us directly (which they have repeatedly done), not just check some field on their govt filings.
Between trusting a company on what they claim and what they actually report, I'd go with what they report. That's what's legally binding. Not that the final result is much different, cash isn't an obstacle in whatever Apple wants to do (and hasn't been in quite some time).
Financial reports use phrases that sound like common sense things but actually have precisely defined meanings that are not equivalent to what they sound like they mean. Basically it's excluding a great deal of Apple's liquid assets because they don't fall under a specific definition.
Total cash, cash equivalents, and marketable securities as of 9/25/2010 = $51.011 billion.
"Start with a billion, then start an airline company"
For Apple, it's probably quite advantageous to be able to essentially pay cash up front for next year's supply of a critical component, pay a partner to build a new plant, etc.
At least Apple isn't throwing their money away in stock buybacks like Ballmer.
I am expecting an iPad price drop on the next version or an iPhone strategy (cheaper old bottom, new bottom, new top). This money allows for the iPod strategy to repeat with the iPad.
Given that, it's probably in their best interest to keep most of it fairly liquid so they can move on product ideas and acquisitions.