Are post tax contribtions to my 401k "rollable" into a roth IRA at the time I leave the company?

No...

Of course, the only thing "rollable" into a Roth IRA is a traditional IRA. But if you have a 401(k) account and have left the company, everything in the 401(k) account except your after-tax contributions can be transferred into a traditional IRA.

...or am I stuck with having to put these funds into a taxable acct?

Yes, but how are you "stuck"?

Since i'm 30 yrs from retirement this could hurt!

Don't really see how. The earnings on those after-tax contributions will be growing, tax-deferred, within the 401(k) all that time, and you will be able to roll those earnings into an IRA if you choose.

And the post-tax contributions will come to you as cash, to do with as you choose -- trip around the world, new sports car, whatever --

Or, you could invest them in a tax-managed mutual fund, or a portfolio of non-dividend-paying stocks and hold them for 20 years, then cash them in and pay only at the long-term capital gains rate on any earnings!

What could be better than that?

Just curious,
Phooley