So I've been running on the Rackspace cloud for a week or so.

The server stays up:

w
22:45:08 up 4 days, 17:23,  1 user,  load average: 0.34, 0.10, 0.10
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU WHAT
root     pts/0    cpe-XXX-XXX-XXX- 22:45    0.00s  0.01s  0.00s w

And it uses memory but can also page some of it to swap if necessary

free -m
             total       used       free     shared    buffers     cached
Mem:           496        275        220          0         11         51
-/+ buffers/cache:        212        284
Swap:         1023         25        998

I must say getting a Apache+mod_fcgid+mod_security+SuExec+PHP5+mysql setup going takes a big chunk of time to do from scratch. But it's interesting! If you charged you own time out to do it you would probably be uneconomic and might better off paying for a managed service but hey if you "get your geek on" then you can learn alot about creating a shared hosting setup.

The Rackspace offering seems solid.

They have a good knowledge base. Knowing from Rackspace KB documents that Cloud Servers aren't trusted as SMTP servers I have configured the local postfix instance to relay through Gmail which seems to work fine. You also get a mail archive in the Sent Mail folder of the Gmail account which may or may not be a good thing.