Is using a generator to improvise a bass line any more "cheating" then having a bass player friend stop by to improvise a bass line for your new tune?  If you play keys, do you need to learn to play the bass, or write the part for your friend?

RapidComposer will do this. It may be a little pricey though. It basically generates midi you can use in you DAW. And it lets you load VSTi's. It's a great tool to get you started.

http://www.musicdevelopments.com/

I tried the RapidComposer demo, and found the UI a bit non-standard and it seemed to take a lot of clicking around to access the tools.  With a DAW-like learning curve, there is lots of potential there if you are willing to put in the time.
 
Ample Sound has recently added the Ample Bass Riffer module to their line of sampled basses.  I tried the demo of the P-Bass and was hooked.  Simple to use, lots of fun, and over 700 bass riffs to play with.  MIDI out too.  
http://www.amplesound.net/en/index.asp
 
Tracktion Waveform 8 has a pattern generator in their PRV.  Easy to come up with chord progressions that work.
https://www.tracktion.com/products/waveform

 
Also interesting is Sundog Scale Studio.  They supposedly have a new version 3 coming out next week.  "Use Sundog to play around with chords and basenotes like a breeze. Go from calm, smooth, chill out styles to energetic upbeat tunes in a matter of seconds."  http://feelyoursound.com/sundog/
 
And then there is Cthulhu.  It is a chord player and pattern based arp module from Xfer Records.
https://www.xferrecords.com/products/cthulhu